Python: Get rid of points-to from Definitions.qll

Turns out the `ImportTime` module (despite living in
`semmle.python.types` does not actually depend on points-to, so some of
the `LegacyPointsTo` imports could be replaced or removed.

python/ql/test/library-tests/PointsTo/general/GlobalPointsTo.ql

@@ -1,5 +1,6 @@
 import python
 private import LegacyPointsTo
+private import semmle.python.types.ImportTime
 import interesting
 
 from int line, ControlFlowNodeWithPointsTo f, Object o, ImportTimeScope n