Port KPluginTrader away from Q_GLOBAL_STATIC.

Summary:
It's just simpler this way, we don't need the extra features
of Q_GLOBAL_STATIC.

Test Plan: Builds

Reviewers: aacid

Reviewed By: aacid

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25816