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/Class.qll

@@ -147,9 +147,6 @@ class Class extends Class_, Scope, AstNode {
   /** Gets a base of this class definition. */
   Expr getABase() { result = this.getParent().getABase() }
 
-  /** Gets the metrics for this class */
-  ClassMetrics getMetrics() { result = this }
-
   /**
    * Gets the qualified name for this class.
    * Should return the same name as the `__qualname__` attribute on classes in Python 3.