C++: Always remove NumericLiterals when removing the AST.

Contrary to StringLiterals, keeping them around is unnecessary.

Change-Id: Idc1967c125e1373e69ce4c7640328d323a84ec14
Reviewed-on: http://codereview.qt.nokia.com/3395


Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>