core: Remove GetEventSubscribersBySubscriberId

GetEventSubscribersBySubscriberIdQuery is not used anywhere in
the code, and hence should be removed.

This patch removes the following:
1. The GetEventSubscribersBySubscriberIdQuery class.
2. The GetEventSubscribersBySubscriberId constant representing
   it in VdcQueryType.

Change-Id: Ib58c99f7e73ac7cd3baff5fe6f53955c328e1c9b
Signed-off-by: Allon Mureinik <amureini@redhat.com>