The yyerror stand-alone function was used to bounce from glr.c's call
to yyerror to glr.cc's parser.error. Now that glr.c is out of the
way, just directly use parser.error.
* data/skeletons/glr2.cc (yyerror): Remove.
Adjust callers.
(b4_yyerror_args, b4_lyyerror_args, b4_pure_formals): Remove.
Now unused.