rbd: don't query snapshot features

Since infernalis, ceph.git commit 281f87f9ee52 ("cls_rbd: get_features
on snapshots returns HEAD image features"), querying and checking that
is pointless.  Userspace support for manipulating image features after
image creation came also in infernalis, so a snapshot with a different
set of features wasn't ever possible.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>