xfs: remove unnecesary lshift/rshift key initialization

In the lshift/rshift functions we don't use the key variable for
anything now, so remove the variable and its initializer.  The
update_keys functions figure out the key for a block on their own.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>