It isn't possible to remove a host network interface using the RESTAPI. Add a host with two network interfaces and create a bond using the GUI, then try to delete the bond using the following script:

---8<---
#!/bin/sh -ex

url="https://engine.example.com/ovirt-engine/api"
user="admin@internal"
password="..."
host_id="..."
nic_id="..."

curl \
--verbose \
--insecure \
--user "${user}:${password}" \
--request DELETE \
"${url}/hosts/${host_id}/nics/${nic_id}"
--->8---

It will fail and return the following error message:

---8<---
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<fault>
    <reason>Operation Failed</reason>
    <detail>[Unexpected exception]</detail>
</fault>
--->8---

The engine.log file contains the following messages:

---8<---
2015-06-09 11:46:28,352 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.RemoveNetworkVDSCommand] (ajp--127.0.0.1-8702-1) [7ec260ee] Failed in 'RemoveNetworkVDS' method
2015-06-09 11:46:28,355 ERROR [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (ajp--127.0.0.1-8702-1) [7ec260ee] Correlation ID: null, Call Stack: null, Custom Event ID: -1, Message: VDSM 'myhost' command failed: Attempt to call function: <bound method Global.delNetwork of <API.Global object at 0x7f73d414afd0>> with arguments: (None, u'bond0', [u'eth1', u'eth0'], None) error: expected string or buffer
2015-06-09 11:46:28,356 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.RemoveNetworkVDSCommand] (ajp--127.0.0.1-8702-1) [7ec260ee] Command 'org.ovirt.engine.core.vdsbroker.vdsbroker.RemoveNetworkVDSCommand' return value 'StatusOnlyReturnForXmlRpc [mStatus=StatusForXmlRpc [mCode=-32603, mMessage=Attempt to call function: <bound method Global.delNetwork of <API.Global object at 0x7f73d414afd0>> with arguments: (None, u'bond0', [u'eth1', u'eth0'], None) error: expected string or buffer]]'
2015-06-09 11:46:28,356 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.RemoveNetworkVDSCommand] (ajp--127.0.0.1-8702-1) [7ec260ee] HostName = myhost
2015-06-09 11:46:28,356 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.RemoveNetworkVDSCommand] (ajp--127.0.0.1-8702-1) [7ec260ee] Command 'RemoveNetworkVDSCommand(HostName = myhost, NetworkVdsmVDSCommandParameters:{runAsync='true', hostId='9889b311-65e4-4df8-8785-bdc7c9b314d2', networkName='ovirtmgmt', oldNetworkName='null', hostAddr='null', checkConnectivity='false', connectionTimeout='0', vlanId='null', bondName='bond0', nics='[eth1, eth0]', inetAddr='null', networkMask='null', gateway='null', stp='false', bondingOptions='null', bootProtocol='NONE', vmNetwork='false', network='null'})' execution failed: VDSGenericException: VDSErrorException: Failed to RemoveNetworkVDS, error = Attempt to call function: <bound method Global.delNetwork of <API.Global object at 0x7f73d414afd0>> with arguments: (None, u'bond0', [u'eth1', u'eth0'], None) error: expected string or buffer, code = -32603
2015-06-09 11:46:28,357 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.RemoveNetworkVDSCommand] (ajp--127.0.0.1-8702-1) [7ec260ee] FINISH, RemoveNetworkVDSCommand, log id: 3fb82615
2015-06-09 11:46:28,358 ERROR [org.ovirt.engine.core.bll.network.host.RemoveBondCommand] (ajp--127.0.0.1-8702-1) [7ec260ee] Command 'org.ovirt.engine.core.bll.network.host.RemoveBondCommand' failed: VdcBLLException: org.ovirt.engine.core.vdsbroker.vdsbroker.VDSErrorException: VDSGenericException: VDSErrorException: Failed to RemoveNetworkVDS, error = Attempt to call function: <bound method Global.delNetwork of <API.Global object at 0x7f73d414afd0>> with arguments: (None, u'bond0', [u'eth1', u'eth0'], None) error: expected string or buffer, code = -32603 (Failed with error unexpected and code 16)
2015-06-09 11:46:28,371 ERROR [org.ovirt.engine.core.bll.network.host.RemoveBondCommand] (ajp--127.0.0.1-8702-1) [7ec260ee] Transaction rolled-back for command 'org.ovirt.engine.core.bll.network.host.RemoveBondCommand'.
2015-06-09 11:46:28,414 ERROR [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (ajp--127.0.0.1-8702-1) [7ec260ee] Correlation ID: 7ec260ee, Job ID: f4b1b049-988b-41e0-9df8-bfd324c0e3f3, Call Stack: null, Custom Event ID: -1, Message: Failed to remove bond: bond0 for Host: myhost (User:admin@internal).
2015-06-09 11:46:28,455 ERROR [org.ovirt.engine.api.restapi.resource.AbstractBackendResource] (ajp--127.0.0.1-8702-1) [] Operation Failed: [Unexpected exception]
--->8---

Apparently there is a mismtach between the parameters sent from the engine, as defined in IVdsServer#delNetwork, and VDSM API.py#delNetwork: 

  Engine: StatusOnlyReturnForXmlRpc delNetwork(String bridge, String vlan, String bond, String[] nics);

  VDSM: def delNetwork(self, network, vlan=None, bond=None, nics=None, options=None):

Note that the VDSM method has an extra "options" parameter.