btrfs: ctree: Remove stray comment of setting up path lock

The following comment shows up in btrfs_search_slot() with out much
sense:

	/*
	 * setup the path here so we can release it under lock
	 * contention with the cow code
	 */
	if (cow) {
		/* code touching path->lock[] is far away from here */
	}

This comment hasn't been cleaned up after the relevant code has been
removed.

The original code is introduced in commit 65b51a00
("btrfs_search_slot: reduce lock contention by cowing in two stages"):

  +
  +               /*
  +                * setup the path here so we can release it under lock
  +                * contention with the cow code
  +                */
  +               p->nodes[level] = b;
  +               if (!p->skip_locking)
  +                       p->locks[level] = 1;
  +

But in current code, we have different timing for modifying path lock,
so just remove the comment.

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>