rt: Remove rust_scheduler::rval. Unused

Author: brson

src/rt/rust_scheduler.h

@@ -14,7 +14,6 @@ class rust_scheduler : public kernel_owned<rust_scheduler> {
     array_list<rust_task_thread *> threads;
     randctx rctx;
     const size_t num_threads;
-    int rval;
 
     void create_task_threads();
     void destroy_task_threads();