[flang] Remove old character "cooking" parser combinators that handled Fortran

[flang] Remove old character "cooking" parser combinators that handled Fortran
comments, continuations, &c. that have become obsolete with the use
of the new C++-coded prescanner module.  Clean out members from
ParseState that were used only by cookedNextChar and its sub-parsers.

Original-commit: flang-compiler/f18@41717531e5eb17dc4f090babbeb63745397d80cc
Reviewed-on: https://github.com/flang-compiler/f18/pull/11
Tree-same-pre-rewrite: false