[plastik] Disable problematic text hints

Summary:
Use of "style: Text.Raised" combined with UTF-8 emoji characters
currently crashes deep in font rendering code.

A report has been submitted upstream:
https://bugreports.qt.io/browse/QTBUG-82311

Given this crash is surprisingly common, this patch removes the text
shadow. I normally would be against removing features, but this was a
subtle effect anyway and frankly just made things look blurry. I doubt
it'll be missed.

BUG: 417699
BUG: 413179

Test Plan:
Made unit test for the crash in qmlscene
Fixed the issue there

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27488