Remove stale declaration for modular GC

Merged: https://github.com/ruby/ruby/pull/12546

internal/gc.h

@@ -255,9 +255,6 @@ void ruby_sized_xfree(void *x, size_t size);
 const char *rb_gc_active_gc_name(void);
 int rb_gc_modular_gc_loaded_p(void);
 
-#if USE_MODULAR_GC
-void ruby_load_modular_gc_from_argv(int argc, char **argv);
-#endif
 RUBY_SYMBOL_EXPORT_END
 
 int rb_ec_stack_check(struct rb_execution_context_struct *ec);

main.c

@@ -24,9 +24,6 @@
 #ifdef HAVE_LOCALE_H
 #include <locale.h>
 #endif
-#if USE_MODULAR_GC
-#include "internal/gc.h"
-#endif
 
 #if defined RUBY_DEVEL && !defined RUBY_DEBUG_ENV
 # define RUBY_DEBUG_ENV 1