Bug 1774130

Proxy user/password/port does not remove after select the 'Http Proxy  = No Http Proxy'

Steps:
 1. Create a Repo with 'Http Proxy' Global Default/Specific Repo
 2. Now edit repo and set 'Http Proxy  = No Http Proxy'

Actual result:

Proxy user/password/port does not remove from that repo:
<pre>

>> /var/log/messages

Oct 30 08:05:34 centos7-luna-devel pulp: celery.app.trace:INFO: [499a55a7] Task pulp.server.managers.repo.importer.update_importer_config[499a55a7-7a45-43a4-881b-a8061d2f8bc3] succeeded in 0.0252606829999s: {'repo_id': '451c3e93-4432-4e67-bd7f-2ac47b1c8201', 'last_updated': datetime.datetime(2019, 10, 30, 8, 5, 34, 871429, tzinfo=<isodate.tzinfo.Utc object at 0x7f8572c7f090>), '_href': '/v2/repositories/451c3e93-4432-4e67-bd7f-2ac47b1c8201/importers/yum_importer/', '_ns': 'repo_importers', 'importer_type_id': 'yum_importer', 'last_override_config': {}, 'last_sync': None, 'scratchpad': None, '_id': ObjectId('5db944073d64f625e1958dec'), 'config': {'proxy_port': 8080, 'download_policy': 'immediate', 'remove_missing': True, 'proxy_username': 'admin', 'proxy_host': '', 'ssl_validation': True, 'proxy_password': '*****'}, 'id': 'yum_importer'}

>> Check repos details using pulp-admin

[root@centos7-luna-devel log]# pulp-admin -u admin -p $pulpAdminPassword repo list --details --repo-id 451c3e93-4432-4e67-bd7f-2ac47b1c8201
+----------------------------------------------------------------------+
                              Repositories
+----------------------------------------------------------------------+

Id:                   451c3e93-4432-4e67-bd7f-2ac47b1c8201
Display Name:         Repo_test_hhtp
Description:          None
Content Unit Counts:  
Notes:                
Scratchpad:           
Importers:            
  Config:               
    Download Policy: immediate
    Proxy Host:                                      <======================
    Proxy Password:  *****                           <======================
    Proxy Port:      8080                            <======================
    Proxy Username:  admin                           <======================
    Remove Missing:  True
    SSL Validation:  True

</pre>

Expected result

Proxy user/password/port should remove from that repo.