core: Delete DbUserBase

This class is empty, and no one is using it, so delete it and have
sub-classes extend IVdcQueryable directly (which is Serializable, so no
need to explicitly implement Serializable).

Change-Id: I9d1fbd4dcf4d543706cdd33ccc49505281541f84
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>