[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