Remove dead rustboot code.

src/rt/rust_task.h

@@ -56,18 +56,9 @@ rust_task : public maybe_proxy<rust_task>,
     ~rust_task();
 
     void start(uintptr_t exit_task_glue,
-               uintptr_t spawnee_abi,
                uintptr_t spawnee_fn,
                uintptr_t args,
                size_t callsz);
-    void start_rustboot(uintptr_t exit_task_glue,
-                        uintptr_t spawnee_fn,
-                        uintptr_t args,
-                        size_t callsz);
-    void start_rustc(uintptr_t exit_task_glue,
-                     uintptr_t spawnee_fn,
-                     uintptr_t args,
-                     size_t callsz);
     void grow(size_t n_frame_bytes);
     bool running();
     bool blocked();