Remove unused branches from mock module (#106617)

* lambda has a name of __none__, but no async lambda so this branch is not needed

* _get_signature_object only returns None for bound builtins. There are no async builtins so this branch isn't needed

* Exclude  a couple of methods from coverage checking in the downstream rolling backport of mock

Lib/test/test_unittest/testmock/testthreadingmock.py

@@ -11,10 +11,10 @@ threading_helper.requires_working_threading(module=True)
 
 class Something:
     def method_1(self):
-        pass
+        pass  # pragma: no cover
 
     def method_2(self):
-        pass
+        pass  # pragma: no cover
 
 
 class TestThreadingMock(unittest.TestCase):