Python: Fix query tests

Mostly just adding `private import LegacyPointsTo`. Sometimes getting
rid of other imports that are superceded by that module.

python/ql/src/Classes/UselessClass.ql

@@ -13,6 +13,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 predicate fewer_than_two_public_methods(Class cls, int methods) {
   (methods = 0 or methods = 1) and