isl_ast_codegen.c: remove remnants of code removed before inclusion
The AST generation code has undergone many changes before it was
included in the master branch in
5888ac1 (add support for generating ASTs
from schedule relations, Mon Sep 17 22:11:06 2012 +0200).
Some references to old code got left in by mistake.
We already removed similar comments from isl_ast_build_expr.c
in
19609c1 (isl_ast_build_expr.c: remove remnants of code removed
before inclusion, Sat Sep 29 11:11:19 2012 +0200).
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>