Remove @pyqtSlot for some non-QObject classes

Not sure if I got all, but at least I got the ones which fail
immediately on start.

See #1669.

qutebrowser/app.py

@@ -476,7 +476,6 @@ class Quitter:
         self._shutting_down = False
         self._args = args
 
-    @pyqtSlot()
     def on_last_window_closed(self):
         """Slot which gets invoked when the last window was closed."""
         self.shutdown(last_window=True)

qutebrowser/config/config.py

@@ -33,7 +33,7 @@ import contextlib
 import collections
 import collections.abc
 
-from PyQt5.QtCore import pyqtSignal, pyqtSlot, QObject, QUrl, QSettings
+from PyQt5.QtCore import pyqtSignal, QObject, QUrl, QSettings
 
 from qutebrowser.config import configdata, configexc, textwrapper
 from qutebrowser.config.parsers import ini, keyconf
@@ -94,8 +94,6 @@ class change_filter:  # pylint: disable=invalid-name
             The decorated function.
         """
         if self._function:
-            @pyqtSlot(str, str)
-            @pyqtSlot()
             @functools.wraps(func)
             def wrapper(sectname=None, optname=None):
                 if sectname is None and optname is None:
@@ -108,8 +106,6 @@ class change_filter:  # pylint: disable=invalid-name
                 else:
                     return func()
         else:
-            @pyqtSlot(str, str)
-            @pyqtSlot()
             @functools.wraps(func)
             def wrapper(wrapper_self, sectname=None, optname=None):
                 if sectname is None and optname is None:

qutebrowser/misc/savemanager.py

@@ -67,7 +67,6 @@ class Saveable:
                               save_on_exit=self._save_on_exit,
                               filename=self._filename)
 
-    @pyqtSlot()
     def mark_dirty(self):
         """Mark this saveable as dirty (having changes)."""
         log.save.debug("Marking {} as dirty.".format(self._name))