Fixup Back/Forward short-cuts

Remove backspace short-cut, Chromium will already handle that if it is
standard on the platform. Also we were not triggering on Back/Forward
buttons due to those being mapped to prev/next tab item in Qt.

Task-number: QTBUG-54546
Change-Id: I9a4f48c718c5685ca9ca1b032d8b04409ac622ca
Reviewed-by: Jesus Fernandez <jesus.fernandez@theqtcompany.com>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>