Remove unused cookiejar NetworkManager, fix parenting.

Before, the QWebPage got passed to the QNetworkAccessManager as cookiejar,
causing no parent to be set. Now, the QWebPage actually owns the
QNetworkAccessManager.

qutebrowser/network/networkmanager.py

@@ -44,7 +44,7 @@ class NetworkManager(QNetworkAccessManager):
                           schemes.
     """
 
-    def __init__(self, cookiejar=None, parent=None):
+    def __init__(self, parent=None):
         log.init.debug("Initializing NetworkManager")
         super().__init__(parent)
         self._requests = []