xfs: remove unused tr_swrite

tr_swrite is never used, remove it.

From a very quick look, I think the usage of it (and its ancestor
XFS_SWRITE_LOG_RES) went away in commit 13e6d5cd "xfs: merge fsync
and O_SYNC handling" back in 2009.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>