Remove the xPath argument from runJavaScript

This value is only used internally by Chromium and risks being removed
at anytime, for which we would then have to maintain a downstream
implementation.

Chromium also only supports frame values, while a complete support of
the interface would require also supporting this xPath to point to
individual document element by affecting the value of "this", the same
way that QWebElement::evaluateJavaScript allowed in QtWebKit.

Change-Id: Id0cb1b8e3bdf9a6db0ca786fb5eb46ffd726d165
Reviewed-by: Michael Bruning <michael.bruning@digia.com>