[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