core: Remove unused methods from UserCommandBase

Removed ProcessAdElementDefaultRole(Guid, String) and
CanAttachVmTo(Guid, ArrayList<String>) from UserCommandBase, as they are
not used anywhere within the codebase.

Removing these methods also allows the removal of the
USER_CANNOT_ATTACH_TO_VM_IN_POOL error code, as it's now no longer
called from anywhere in the codebase.

Change-Id: I175379ee180a4c950a355c990b68cfd588dd7f8b
Signed-off-by: Allon Mureinik <amureini@redhat.com>