xfs: drop unused ioend private merge and setfilesize code

XFS no longer attaches anthing to ioend->io_private. Remove the
unnecessary ->io_private merging code. This removes the only remaining
user of xfs_setfilesize_ioend() so remove that function as well.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>