Get rid of rust_crate_cache in the runtime

We are no longer generating dynamic tydescs or dicts.

Issue #1982

src/rt/rust_task_thread.cpp

@@ -24,7 +24,6 @@ rust_task_thread::rust_task_thread(rust_scheduler *sched,
                                    int id) :
     rust_thread(SCHED_STACK_SIZE),
     _log(srv, this),
-    cache(this),
     id(id),
     should_exit(false),
     cached_c_stack(NULL),
@@ -295,11 +294,6 @@ rust_task_thread::start_main_loop() {
     }
 }
 
-rust_crate_cache *
-rust_task_thread::get_cache() {
-    return &cache;
-}
-
 rust_task *
 rust_task_thread::create_task(rust_task *spawner, const char *name,
                             size_t init_stack_sz) {