[Qt] Use WebPageGroup to create user scripts instead of Qt-specific function in WebPage
https://bugs.webkit.org/show_bug.cgi?id=105921

Reviewed by Simon Hausmann.

After r131281 the WebPageGroup.h (in UIProcess) exposes a way to add user scripts, so we can
use this directly to implement the user script functionality in Qt. The previous solution was
a Qt-port specific function in WebPage that can be removed now.

* UIProcess/API/qt/qquickwebview.cpp:
(QQuickWebViewPrivate::updateUserScripts): Use WebPageGroup functions to manipulate user scripts directly.
* UIProcess/WebPageProxy.h:
(WebPageProxy): Remove now unused function.
* UIProcess/qt/WebPageProxyQt.cpp: Ditto.
* WebProcess/WebPage/WebPage.messages.in: Ditto.
* WebProcess/WebPage/qt/WebPageQt.cpp: Remove now unused function and headers.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138621 268f45cc-cd09-0410-ab3c-d52691b4dbfc