restapi: remove unneeded fetch of template disk entity

Remove unneeded call to AbstractBackendCollectionResource#getEntity from
BackendTemplateDisksResource#performRemove, as this method is already
called from AbstractBackendCollectionResource#remove before to ensure
the entity exists, and that's the only flow in which
BackendTemplateDisksResource#performRemove should be called.

Change-Id: Iae1f12ad70371127fd4dceddf302798fa8b750b4
Signed-off-by: Arik Hadas <ahadas@redhat.com>