Remove CombinedRequest and BrowserDetails (#8165)

Remove CombinedRequest and BrowserDetails (#8165)

* Primary use case for CombinedRequest (special path and parameters)
already elimiated by other changes
* BrowserDetails.getLocation is now available through Page
* BrowserDetails.getWindowName only used internally in one location
* VaadinServletRequest.cast and similar for portlets removed now that a
normal cast can always be used as there's no CombinedRequest to consider

Change-Id: I44f28722a12f86015b3c30e83768e4611b87479c