btrfs: remove no longer used counter when reading data page

After commit 92082d40976ed0 ("btrfs: integrate page status update for
data read path into begin/end_page_read"), the 'nr' counter at
btrfs_do_readpage() is no longer used, we increment it but we never
read from it. So just remove it.

Reviewed-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: Filipe Manana <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/extent_io.c

···
 	u64 cur_end;
 	struct extent_map *em;
 	int ret = 0;
-	int nr = 0;
 	size_t pg_offset = 0;
 	size_t iosize;
 	size_t blocksize = inode->i_sb->s_blocksize;
···
 					 end_bio_extent_readpage, 0,
 					 this_bio_flag,
 					 force_bio_submit);
-		if (!ret) {
-			nr++;
-		} else {
+		if (ret) {
 			unlock_extent(tree, cur, cur + iosize - 1);
 			end_page_read(page, false, cur, iosize);
 			goto out;