magit-wip-save: remove unneeded kludge

(This is a duplicate of pull request #477. I needed to recreate it because I used the next branch for that pull request and I want to be able to use next properly again.)

(Also: Could someone please merge this soonish. I am sure this change won't cause any problems: I wrote (1) the function being modified (2) the kludge I now want removed and (3) the proper fix which makes the kludge unnecessary :-)

A proper fix for this issue (#441) has been merged.

> from the branch you target for final inclusion. This should always be either maint or master.

I suppose *always* mean *except when it depends on commits already in next* :-)