Remove request tracking from NetworkManager

This breaks things (with "ValueError: list.remove(x): x not in list") on
PyQt 5.9 (probably due to the destroyed object tracking it introduces?).

This was originally added in 0abb5cf to fix
some segfaults on exit, but things look much better with recent Qt versions.

qutebrowser/browser/webkit/network/networkmanager.py

@@ -121,7 +121,6 @@ class NetworkManager(QNetworkAccessManager):
                            reparented to the DownloadManager. This counts the
                            still running downloads, so the QNAM can clean
                            itself up when this reaches zero again.
-        _requests: Pending requests.
         _scheme_handlers: A dictionary (scheme -> handler) of supported custom
                           schemes.
         _win_id: The window ID this NetworkManager is associated with.
@@ -146,7 +145,6 @@ def __init__(self, *, win_id, tab_id, private, parent=None):
         self.adopted_downloads = 0
         self._win_id = win_id
         self._tab_id = tab_id
-        self._requests = []
         self._private = private
         self._scheme_handlers = {
             'qute': webkitqutescheme.QuteSchemeHandler(win_id),
@@ -201,9 +199,6 @@ def _get_abort_signals(self, owner=None):
     def shutdown(self):
         """Abort all running requests."""
         self.setNetworkAccessible(QNetworkAccessManager.NotAccessible)
-        for request in self._requests:
-            request.abort()
-            request.deleteLater()
         self.shutting_down.emit()
 
     # No @pyqtSlot here, see
@@ -370,9 +365,6 @@ def set_referer(self, req, current_url):
     def createRequest(self, op, req, outgoing_data):
         """Return a new QNetworkReply object.
 
-        Extend QNetworkAccessManager::createRequest to save requests in
-        self._requests and handle custom schemes.
-
         Args:
              op: Operation op
              req: const QNetworkRequest & req
@@ -437,6 +429,4 @@ def createRequest(self, op, req, outgoing_data):
                 reply = super().createRequest(op, req, outgoing_data)
         else:
             reply = super().createRequest(op, req, outgoing_data)
-        self._requests.append(reply)
-        reply.destroyed.connect(self._requests.remove)
         return reply