Btrfs: cleanup for duplicated code in find_free_extent

There is already an 'add free space' phrase in front of this one, we
needn't to redo it.
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>

fs/btrfs/extent-tree.c

@@ -5828,10 +5828,6 @@ static noinline int find_free_extent(struct btrfs_trans_handle *trans,
 
 		trace_btrfs_reserve_extent(orig_root, block_group,
 					   search_start, num_bytes);
-		if (offset < search_start)
-			btrfs_add_free_space(used_block_group, offset,
-					     search_start - offset);
-		BUG_ON(offset > search_start);
 		if (used_block_group != block_group)
 			btrfs_put_block_group(used_block_group);
 		btrfs_put_block_group(block_group);