Revert "Merge pull request #30255 from bkchr/qt_5_9_2"

This reverts commit 4cf0b6ba, reversing
changes made to 756b9c7d.

Qt 5.9.2 is reverted to Qt 5.9.1, as recommended by KDE upstream due to
https://bugreports.qt.io/browse/QTBUG-64017