populate_db: Clean up existing RemoteRealm, RemoteZulipServer objects.

As of c9b0602320 and
8b55d60f9e we create a default
registration for the dev env "RemoteZulipServer" (and also RemoteRealms
for the dev realms) in populate_db. However these models weren't listed
in clear_database, meaning the state wasn't properly cleaned up at the
start of the command.

Most important, this would manifest in getting:
```
django.db.utils.IntegrityError: duplicate key value violates unique
constraint "zilencer_remotezulipserver_uuid_key"
DETAIL:  Key (uuid)=(......) already exists
```

if you re-run populate_db.

zilencer/management/commands/populate_db.py

@@ -73,7 +73,7 @@ from zerver.models import (
     get_user_by_delivery_email,
     get_user_profile_by_id,
 )
-from zilencer.models import RemoteZulipServer
+from zilencer.models import RemoteRealm, RemoteZulipServer
 from zilencer.views import update_remote_realm_data_for_server
 
 settings.USING_TORNADO = False
@@ -128,6 +128,8 @@ def clear_database() -> None:
         UserMessage,
         Client,
         DefaultStream,
+        RemoteRealm,
+        RemoteZulipServer,
     ]:
         model.objects.all().delete()
     Session.objects.all().delete()