Python: Get rid of getLiteralValue

This had only two uses in our libraries, so I simply inlined the
predicate body in both places.

python/ql/lib/semmle/python/Exprs.qll

@@ -337,8 +337,6 @@ abstract class ImmutableLiteral extends Expr {
   abstract Object getLiteralObject();
 
   abstract boolean booleanValue();
-
-  final Value getLiteralValue() { result.(ConstantObjectInternal).getLiteral() = this }
 }
 
 /** A numerical constant expression, such as `7` or `4.2` */