Shib: Ensure browser view gets closed before QNAM

Otherwise, QWebPage tries to delete its QNetworkReplies, which have
already been deleted by the QNAM destruction, who is their parent.

Fixes #1840
Shib: remove slotLoadFinished() from webview dtor

This is a relict from when the webview was reused.
It is now pointless, since it's now destruct-on-close.