xfs: remove the all-mounts list

Revert commit 0ed17f01 ("xfs: introduce all-mounts list for cpu
hotplug notifications") because the cpu hotplug hooks are now pointless,
so we don't need this list anymore.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>