Remove unnecessary checks from SetupState

Since 4622c28ffd, _finalizers cannot
contain Nones or tuples, so these checks are not longer needed.

src/_pytest/runner.py

@@ -278,10 +278,7 @@ class SetupState:
         self._finalizers = {}
 
     def addfinalizer(self, finalizer, colitem):
-        """ attach a finalizer to the given colitem.
-        if colitem is None, this will add a finalizer that
-        is called at the end of teardown_all().
-        """
+        """ attach a finalizer to the given colitem. """
         assert colitem and not isinstance(colitem, tuple)
         assert callable(finalizer)
         # assert colitem in self.stack  # some unit tests don't setup stack :/
@@ -309,12 +306,9 @@ class SetupState:
 
     def _teardown_with_finalization(self, colitem):
         self._callfinalizers(colitem)
-        if hasattr(colitem, "teardown"):
-            colitem.teardown()
+        colitem.teardown()
         for colitem in self._finalizers:
-            assert (
-                colitem is None or colitem in self.stack or isinstance(colitem, tuple)
-            )
+            assert colitem in self.stack
 
     def teardown_all(self):
         while self.stack: