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/Module.qll

@@ -86,9 +86,6 @@ class Module extends Module_, Scope, AstNode {
     result = this.getName().regexpReplaceAll("\\.[^.]*$", "")
   }
 
-  /** Gets the metrics for this module */
-  ModuleMetrics getMetrics() { result = this }
-
   string getAnImportedModuleName() {
     exists(Import i | i.getEnclosingModule() = this | result = i.getAnImportedModuleName())
     or