magit-log-toggle-margin: don't call magit-refresh

`magit-log-toggle-margin` is pretty useful, but I notice it calls `magit-refresh` when re-enabling the margin. The pause makes it kind of annoying to use. 

The refresh call doesn't seem needed, it seems to work without it. I'm not sure if the old code sometimes toggling on `(cdr (window-margins))` instead of `magit-log-show-margin` is important, I've dropped that behaviour...