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/lib/analysis/DefinitionTracking.qll

@@ -4,6 +4,7 @@
 
 import python
 private import LegacyPointsTo
+private import semmle.python.types.ImportTime
 import IDEContextual
 
 private newtype TDefinition =