[effects/minimizeanimation] Delete useless comments

Summary:
Comments like "// Call the next effect." are sort of

    // Add 1 to foo.
    foo += 1

they don't give any useful information and it would be better without
them.

Test Plan: Still compiles.

Reviewers: #kwin

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14728