btrfs: remove the incorrect comment on RO fs when btrfs_run_delalloc_range() fails

At the context of btrfs_run_delalloc_range(), we haven't started/joined
a transaction, thus even something went wrong, we can't and won't abort
transaction, thus no way to make the fs RO.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -3310,7 +3310,6 @@ static noinline_for_stack int writepage_delalloc(struct inode *inode,
 		}
 		ret = btrfs_run_delalloc_range(inode, page, delalloc_start,
 				delalloc_end, &page_started, nr_written, wbc);
-		/* File system has been set read-only */
 		if (ret) {
 			SetPageError(page);
 			/*