[Bookmarks Runner] Fix cleanup of favicon directory

Summary:
QDir.entryInfoList(NoDotAndDotDot) always returns an empty list, as it
requires one of QDir::Files, QDir::Dirs or QDir::Drives or any
combination thereof.

Instead of implementing it manually, use QDir::removeRecursively().

Test Plan:
Watch ~/.cache/krunner/
When krunner is closed, KRunner-Favicons-firefox-default should
be emptied and deleted.

Reviewers: #plasma, davidedmundson, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15391