Draft: get rid of xkb workarounds

Xkb::toQtKey() was solely existed to hold workarounds for Qt key mappings.
Now when all of those was fixed in upstream Qt, we can get rid of the unneeded function.

Relevant Qt patches:
https://codereview.qt-project.org/c/qt/qtbase/+/349549
https://codereview.qt-project.org/c/qt/qtbase/+/350147
https://codereview.qt-project.org/c/qt/qtbase/+/340433