xfs: remove a stale comment from xfs_file_aio_write_checks()

The comment in xfs_file_aio_write_checks() about calling file_modified()
after dropping the ilock doesn't make sense, because the code that
unconditionally acquires and drops the ilock was removed by
commit 467f7899

 ("xfs: reduce ilock hold times in
xfs_file_aio_write_checks").

Remove this outdated comment.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>