btrfs: relocation: Remove redundant tree level check

Commit 581c1760 ("btrfs: Validate child tree block's level and first
key") has made tree block level check mandatory.

So if tree block level doesn't match, we won't get a valid extent
buffer.  The extra WARN_ON() check can be removed completely.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -2915,7 +2915,6 @@ static int get_tree_block_key(struct btrfs_fs_info *fs_info,
 		free_extent_buffer(eb);
 		return -EIO;
 	}
-	WARN_ON(btrfs_header_level(eb) != block->level);
 	if (block->level == 0)
 		btrfs_item_key_to_cpu(eb, &block->key, 0);
 	else