xfs: remove unused XBT_FORCE_SLEEP bit

XBT_FORCE_SLEEP is no longer ever tested; it is only set
and cleared.  Remove it.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>