core: Removing a storage connection without a VDS

If a VDS ID is not supplied to the
RemoveStorageServerConnectionCommand, it should be possible to remove
the connection from the database without disconnecting it from the
(non-existent) host.

Change-Id: I6ddcf6cb6cd2aed85c23f0e7565ab0b597474a81
Signed-off-by: Allon Mureinik <amureini@redhat.com>