Remove custom menu positioning code
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
Differential Revision: https://phabricator.kde.org/D23142