[effects] Drop static_cast in getMainWindows
Summary:
We use a function template, so static_cast in getMainWindows looks out
of place. Also, it's not good idea to do static_cast based on output of
isDeleted().
Test Plan: The Dialog Parent effect works.
Reviewers: KWin, graesslin
Reviewed By: KWin, graesslin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D17426