Remove unnecessary qguiapplication_p.h include

The include was previously used for implementing
QWebEnginePagePrivate::unhandledKeyEvent which called
QGuiApplicationPrivate::instance()->shortcutMap.tryShortcut(event).
This logic has now been moved to render_widget_host_view_qt.cpp.
For now, at least remove it from this file.

Change-Id: I516a659b2fb21782aabac7e7de8c1f93d0dbef91
Task-number: QTBUG-63098
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>