make mc/mark-next-like-this with nagative ARG to delete a cursor even if the region is not active.

```mc/mark-next-like-this``` (```mc/mark-previous-like-this```) does nothing with a nagative argument when the region is not active.
I think it should remove a cursor regardless of the region activity
because we have no means to delete a cursor if the region is not active.
What do you think about it?