Workaround crash in Aurorae destruction

Summary:
Workaround QtBug-68997

Deleting of a RenderControl/render controlled window triggers deletion
of other queued deleted items, putting KWin in a corrupt state. See Qt
bug report.

Deleting this queued means we know we don't have anything else going on
in the stack which should make this somewhat safe.

BUG: 395346

Test Plan:
Couldn't reproduce original crash. Based purely on the incomplete backtrace and code reading

Set an Aurorae theme. Closed some windows. Things still worked as before

Reviewers: KWin, Plasma, mart

Reviewed By: KWin, Plasma, mart

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13614