mds: Implement remove for ceph vxattrs

mds: Implement remove for ceph vxattrs

For a variety of ceph xattrs, the setfattr remove
isn't handled and "No such attribute" is returned.

Add support to remove xattrs to match where docs
say to clear value, one can remove or set to X value.
When removing, add "default" value back to xattr depending
on xattr name.

Simplify current code to make clearer and minimize code reuse.

Fixes: https://tracker.ceph.com/issues/62793
Signed-off-by: Christopher Hoffman <choffman@redhat.com>
(cherry picked from commit 1f0a099e4c41f62b55cfa3fd620ac06be34488da)

Conflicts:
        src/mds/Server.cc
          get_ephemeral_distributed_pin() DNE, export_ephemeral_distributed_pin used
        src/mds/Server.h
          dropped const in handle_client_setvxattr