btrfs: Remove never-reached WARN_ON

We have a WARN_ON(!var) inside an if branch which is executed (among
others) only when var is true.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -6745,7 +6745,7 @@ static int unpin_extent_range(struct btrfs_fs_info *fs_info,
 		if (!readonly && return_free_space &&
 		    global_rsv->space_info == space_info) {
 			u64 to_add = len;
-			WARN_ON(!return_free_space);
+
 			spin_lock(&global_rsv->lock);
 			if (!global_rsv->full) {
 				to_add = min(len, global_rsv->size -