MNT: remove registration of dummy PeakWidthModel

Make unit tests exit with a clean registry.

Author: pavoljuhas

src/diffpy/srreal/tests/testpeakwidthmodel.py

@@ -261,12 +261,16 @@ def ticker(self):
         self.tcnt += 1
         return PeakWidthModel.ticker(self)
 
-MyPWM()._registerThisType()
-
 # End of class MyPWM
 
 class TestPeakWidthOwner(unittest.TestCase):
 
+    @classmethod
+    def tearDownClass(cls):
+        assert "mypwm" not in PeakWidthModel.getRegisteredTypes()
+        return
+
+
     def setUp(self):
         self.pc = PDFCalculator()
         self.dbpc = DebyePDFCalculator()