[effects/slide] Delete redundant checks
Summary:
If isActive() returns false, neither paintScreen nor paintWindow nor
postPaintScreen will be called.
So, if (m_active) checks are redundant.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D15708