core: Remove IVdcUser

This interface only has one implementation, and there are no plans to
have more so it can be removed, and that will simplify a bit the user
management in the engine.

Change-Id: I95315c0a31a1da9fbb2f758557bd0ec4ec3fc961
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>