Don't double delete CommentsModel

Summary:
CommentsModel instances are owned by the engine.

They are fetched through a getter with a cache, so ownership has to stay
that way.

QuickItemsModel should not be deleting them.

BUG: 417802

Test Plan:
Got a valgrind log of the crash, which showed the issue
(see attached bug report)

Afterwards:
Opened systemsettings, opened comments dialog
Closed systemsettings

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D27468