Fix QNetworkRequest usage in WebEngineWidgets on Windows

Changing unused variable suppression in acceptNavigationRequest
is required for MSVC otherwise it fail with undefined type error.
Also adding more suppression for unused wariable warning.

Change-Id: Ic449973bc8166f18655e40bf57041b8f02869266
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Cleanup on webenginewidgets.pro

Remove unneeded parts that handled by load(qt_module) already.

Change-Id: I50646d754d1eec96342ba2f7c45f1a023a7257ae
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>