remove visit of available module in dynamic-require

Even though `dynamic-require` might lead to loading source, the
path into the compiler for that source will force compile-time code
as needed.

One benefit of ths change is that `racket -l pict3d` takes about half
as long, because `racket/gui` includes a `dynamic-require` to load a
platform-specific back-end, while `pict3d` can pull in a lot of
compile-time code to cooperate with Typed Racket.

racket/src/racket/src/module.c

@@ -1129,8 +1129,6 @@ static Scheme_Object *_dynamic_require(int argc, Scheme_Object *argv[],
 
   base_phase = env->phase;
 
-  scheme_prepare_compile_env(env);
-
   m = module_load(modname, env, errname);
   srcm = m;