test-client: drop unused NmTestBase base class

The base class is not used, and it's not clear that it would be useful.
Sure, we could extend "test-client.py" will various non-nmcli tests. That
might make sense. And then it might make sense to have more unit test classes.
So far, we don't need that. Drop the unused base class NmTestBase.

(cherry picked from commit d1e6d53013)

src/tests/client/test-client.py

@@ -740,20 +740,18 @@ class AsyncProcess:
 ###############################################################################
 
 
-class NmTestBase(unittest.TestCase):
+MAX_JOBS = 15
+
+
+class TestNmcli(unittest.TestCase):
     def __init__(self, *args, **kwargs):
         self._calling_num = {}
         self._skip_test_for_l10n_diff = []
         self._async_jobs = []
         self._results = []
         self.srv = None
-        return unittest.TestCase.__init__(self, *args, **kwargs)
+        unittest.TestCase.__init__(self, *args, **kwargs)
 
-
-MAX_JOBS = 15
-
-
-class TestNmcli(NmTestBase):
     def srv_start(self):
         self.srv_shutdown()
         self.srv = NMStubServer(self._testMethodName)