Prevent already destroyed widget exception occurring on Slicer exit

This commit avoid the exception reported below from happening when
exiting Slicer:

Exception ValueError: "Trying to call 'deleteLater' on a destroyed qMRMLNodeComboBox object" in <bound method FilterParameters.__del__ of <SimpleFilters.FilterParameters object at 0x5a13390>> ignored

It is not required to explicitly delete the widgets associated with the
last shown filter. The associated widgets have a parent and will be
deleted by the application.

SimpleFilters/SimpleFilters.py

@@ -541,7 +541,7 @@ def __init__(self, parent=None):
     self.outputLabelMapBox = None
 
   def __del__(self):
-    self.destroy()
+    self.widgets = []
 
   def BeautifyCamelCase(self, str):
     return self.reCamelCase.sub(r' \1',str)