Python: Get rid of some get...Object methods

This frees `Class.qll`, `Exprs.qll`, and `Function.qll` from the
clutches of points-to. For the somewhat complicated setup with
`getLiteralObject` (an abstract method), I opted for a slightly ugly but
workable solution of just defining a predicate on `ImmutableLiteral`
that inlines each predicate body, special-cased to the specific instance
to which it applies.

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

@@ -87,9 +87,6 @@ class Function extends Function_, Scope, AstNode {
   /** Gets the metrics for this function */
   FunctionMetrics getMetrics() { result = this }
 
-  /** Gets the FunctionObject corresponding to this function */
-  FunctionObject getFunctionObject() { result.getOrigin() = this.getDefinition() }
-
   /**
    * 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.