Remove D10890 workaround

Summary:
D10890 was a workaround related to KRecentDocument::add() bug (see D10863)
which is fixed in Frameworks 5.46. Given that Plasma 5.13 depends on that
Frameworks version, it's safe to remove this no longer needed workaround
without breaking anything.

Reviewers: #plasma, broulik, hein

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12701