btrfs: Remove impossible BUG_ON in get_tree_block_key

relocate_tree_blocks calls get_tree_block_key for a block iff that block
has its ->key_ready equal false. Thus the BUG_ON in the latter function
cannot ever be triggered so remove it.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -3089,7 +3089,6 @@ static int get_tree_block_key(struct btrfs_fs_info *fs_info,
 {
 	struct extent_buffer *eb;
 
-	BUG_ON(block->key_ready);
 	eb = read_tree_block(fs_info, block->bytenr, block->key.offset,
 			     block->level, NULL);
 	if (IS_ERR(eb)) {