Remove scrolled window container around WebKit widget

Remove scrolled window container around WebKit widget

The WebKit widget can scroll on its own and does not need to wrapped
with a scrolled window container.

* src/xwidget.h: Remove struct member widgetscrolledwindow_osr.
* src/xwidget.c: Remove widgetscrolledwindow_osr.
(xwidget-set-adjustment): Remove.
(xwidget-resize): Resize Webkit widget last.
* lisp/xwidget.el (xwidget-set-adjustment): Remove.
(xwidget-webkit-scroll-up, xwidget-webkit-scroll-down,
xwidget-webkit-scroll-forward, xwidget-webkit-scroll-backward):
Implement scrolling via JavaScript.