btrfs: kill unused writepage_io_hook callback

It seems to be long time unused, since 2008 and
6885f308 ("Btrfs: Misc 2.6.25 updates").

Propagating the removal touches some code but has no functional effect.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3368,6 +3368,8 @@ static noinline_for_stack int __extent_writepage_io(struct inode *inode,
 
 	while (cur <= end) {
 		u64 em_end;
+		unsigned long max_nr;
+
 		if (cur >= i_size) {
 			if (tree->ops && tree->ops->writepage_end_io_hook)
 				tree->ops->writepage_end_io_hook(page, cur,
@@ -3423,32 +3425,23 @@ static noinline_for_stack int __extent_writepage_io(struct inode *inode,
 			continue;
 		}
 
-		if (tree->ops && tree->ops->writepage_io_hook) {
-			ret = tree->ops->writepage_io_hook(page, cur,
-						cur + iosize - 1);
-		} else {
-			ret = 0;
+		max_nr = (i_size >> PAGE_SHIFT) + 1;
+
+		set_range_writeback(tree, cur, cur + iosize - 1);
+		if (!PageWriteback(page)) {
+			btrfs_err(BTRFS_I(inode)->root->fs_info,
+				   "page %lu not writeback, cur %llu end %llu",
+			       page->index, cur, end);
 		}
-		if (ret) {
-			SetPageError(page);
-		} else {
-			unsigned long max_nr = (i_size >> PAGE_SHIFT) + 1;
 
-			set_range_writeback(tree, cur, cur + iosize - 1);
-			if (!PageWriteback(page)) {
-				btrfs_err(BTRFS_I(inode)->root->fs_info,
-					   "page %lu not writeback, cur %llu end %llu",
-				       page->index, cur, end);
-			}
+		ret = submit_extent_page(write_flags, tree, wbc, page,
+					 sector, iosize, pg_offset,
+					 bdev, &epd->bio, max_nr,
+					 end_bio_extent_writepage,
+					 0, 0, 0, false);
+		if (ret)
+			SetPageError(page);
 
-			ret = submit_extent_page(write_flags, tree, wbc, page,
-						 sector, iosize, pg_offset,
-						 bdev, &epd->bio, max_nr,
-						 end_bio_extent_writepage,
-						 0, 0, 0, false);
-			if (ret)
-				SetPageError(page);
-		}
 		cur = cur + iosize;
 		pg_offset += iosize;
 		nr++;

fs/btrfs/extent_io.h

@@ -71,7 +71,6 @@ struct extent_io_ops {
 			     u64 start, u64 end, int *page_started,
 			     unsigned long *nr_written);
 	int (*writepage_start_hook)(struct page *page, u64 start, u64 end);
-	int (*writepage_io_hook)(struct page *page, u64 start, u64 end);
 	extent_submit_bio_hook_t *submit_bio_hook;
 	int (*merge_bio_hook)(int rw, struct page *page, unsigned long offset,
 			      size_t size, struct bio *bio,