Remove experimental GC code

It's been sitting unused long enough to have bitrotted completely.

src/rt/rust_builtin.cpp

@@ -91,11 +91,6 @@ refcount(intptr_t *v) {
     return (*v) - 1;
 }
 
-extern "C" CDECL void
-do_gc() {
-    // TODO
-}
-
 extern "C" CDECL void
 unsupervise() {
     rust_task *task = rust_scheduler::get_task();