(compilation-parse-errors-function): Remove obsolete var

(compilation-parse-errors-function): Remove obsolete var

* lisp/progmodes/compile.el (compilation-parse-errors-function): Remove.
(compilation--parse-region): Simplify accordingly.
(compilation--compat-parse-errors): Remove function, not used any more.
(compilation-error-list, compilation-parsing-end)
(compilation-old-error-list): Remove vars, not used any more.
(compilation-last-buffer): Mark as obsolete (sadly, forgot to do that
back in Emacs-22).

* lisp/textmodes/tex-mode.el: Remove old code that was needed for
Emacs<24.

* lisp/progmodes/prolog.el (prolog-consult-compile): Remove long-dead code.
(compilation-parse-errors-function): Remove declaration, not used any more.
(prolog-parse-sicstus-compilation-errors): Remove function, not used any more.