actually delete old theme caches

Summary:
QStandardPaths doesn't do file search by wildcards,
that code was never actually functional

Test Plan: old kcache and rects cache files get correctly removed

Reviewers: Plasma, sitter

Reviewed By: sitter

Subscribers: davidedmundson, sitter, apol, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4925