core: Del ImagesHandler.isDomainWithinThresholds

ImagesHandler.isDomainWithThresholds is useless method - all it does is
invoke a StorageDomainValidator, which could (and should!) be done
directly.

This patch replaces the single usage it had with a direct call to the
validator, and removes this method, along with the validate method
that's only called from it.

Change-Id: I7a0c776e9798d6128a3aac4400e1a22b3df84b56
Signed-off-by: Allon Mureinik <amureini@redhat.com>