[flang] Removed erroneous reference to "obsolete" and fixed up the message

as per Peter Klauser's recommendation.

Original-commit: flang-compiler/f18@f4d6d7007d6ce77761b0f2b99be011d48a996ce5
Reviewed-on: https://github.com/flang-compiler/f18/pull/398
Tree-same-pre-rewrite: false