w6c: delete unwired backend pass seams

cmd/w6c/gc.h

@@ -73,8 +73,4 @@ void	txt_emit(FILE*, Prog *head);
 /* wwi.c — non-zero return: check_exported_type rejected a dangling export. */
 int	wwi_emit(Checker *c, FILE *of, Node *file);
 
-/* swt.c, peep.c, reg.c — placeholders for now */
-void	peephole(Cg*);
-void	regalloc_init(Cg*);
-
 #endif