xfs: remove xfs_start_page_writeback

This helper only has two callers, one of them with a constant error
argument.  Remove it to make pending changes to the code a little easier.

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