Python: Move metrics-related API to LegacyPointsTo module

Gets rid of the `getMetrics` methods on the `Function`, `Class`, and
`Module` classes. To access the metrics, one must first import the
`LegacyPointsTo` module, and then either change the type to
`{Function,Class,Module}Metrics` or cast to the appropriate type.

python/ql/lib/semmle/python/Function.qll

@@ -84,9 +84,6 @@ class Function extends Function_, Scope, AstNode {
   /** Gets the name used to define this function */
   override string getName() { result = Function_.super.getName() }
 
-  /** Gets the metrics for this function */
-  FunctionMetrics getMetrics() { result = this }
-
   /**
    * Whether this function is a procedure, that is, it has no explicit return statement and always returns None.
    * Note that generator and async functions are not procedures as they return generators and coroutines respectively.