ubifs: Remove unused security xattr handler

Ubifs installs a security xattr handler in sb->s_xattr but doesn't use the
generic_{get,set,list,remove}xattr inode operations needed for processing
this list of attribute handlers; the handler is never called.  Instead,
ubifs uses its own xattr handlers which also process security xattrs.

Remove the dead code.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: linux-mtd@lists.infradead.org
Cc: Subodh Nijsure <snijsure@grid-net.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>