MDS: remove a few other unnecessary is_base() checks

We should let users remove xattrs as well as set them. ;) And
the check in handle_client_setlayout was totally useless -- perhaps
intended for setdirlayout?

This is a follow-on to 9f82ae60fac30391dfa9d17d2fc014bf9e21f387 and
should be taken wherever it goes.

Signed-off-by: Greg Farnum <greg@inktank.com>