remove obsolete rb_gc_finalize_deferred().

rb_gc_finalize_deferred() is remained for compatibility with
C-extensions. However, this function is no longer working
from Ruby 2.4 (crash with SEGV immediately).
So remove it completely.

gc.c

@@ -3140,13 +3140,6 @@ gc_finalize_deferred(void *dmy)
     ATOMIC_SET(finalizing, 0);
 }
 
-/* TODO: to keep compatibility, maybe unused. */
-void
-rb_gc_finalize_deferred(void)
-{
-    gc_finalize_deferred(0);
-}
-
 static void
 gc_finalize_deferred_register(rb_objspace_t *objspace)
 {

include/ruby/intern.h

@@ -520,7 +520,6 @@ VALUE rb_gc_location(VALUE);
 void rb_gc_force_recycle(VALUE);
 void rb_gc(void);
 void rb_gc_copy_finalizer(VALUE,VALUE);
-void rb_gc_finalize_deferred(void);
 void rb_gc_call_finalizer_at_exit(void);
 VALUE rb_gc_enable(void);
 VALUE rb_gc_disable(void);