qtwebengine: remove no-sandbox flag

In 64720224, a no-sandbox flag was
added because QtWebEngine can't load fonts on recent glibc. Source:
https://bugreports.qt.io/browse/QTBUG-92969
https://bugs.chromium.org/p/chromium/issues/detail?id=1164975

Now, we use Qt 6.2, so this flag should not be necessary.
So, we can finally close #442 without any dirty workaround.

GitLab: #442
Change-Id: I52835e40d37d2397a891415fd2b1dd8dd4048b8b