xfs: remove xfs_defer_reset

Remove this one-line helper since the assert is trivially true in one
call site and the rest obscures a bitmask operation.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>