Remove custom menu positioning code

Summary:
Qt will position the menu just fine, this is overhead.
No matter how hard I tried, I couldn't get the menu to show up in a
position where it wasn't shown entirely, removing this should be safe.
Suggested by zzag.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23142