Description of problem:
After update the cgroup property with systemd the value is changed in associated cgroup resource in /sys/fs/cgroup/cpu,cpuacct/system.slice/sshd.service but after set it to -1 value is not updating in cgroup structure.

Version-Release number of selected component (if applicable):

systemd-219-19.el7.x86_64
How reproducible:

Always
Steps to Reproduce:
1) Set property value to 500 for sshd.service after execute the below command 
systemctl set-property sshd.service CPUShares=500
2) Check the value in cgroup file system
cat /sys/fs/cgroup/cpu,cpuacct/system.slice/sshd.service/cpu.shares 
500
3) set the default value to -1
systemctl set-property sshd.service CPUShares=-1
4) Now check again the value in cgroup file system
cat /sys/fs/cgroup/cpu,cpuacct/system.slice/sshd.service/cpu.shares 
500
It is still showing old value even new value is updated in systemd internal structures 
systemctl show properties sshd.service -p CPUShares
CPUShares=18446744073709551615


Actual results:
Value is not reflecting cgroup file system.

Expected results:
It should reflect in cgroup file system and cgroup file structure should also removed after set it to -1.

Additional info: