Remove unimplemented register stack functions

These functions were added for itanium and are no longer used.

- void set_base_of_stack_pointer(intptr_t* base_sp) {}
- intptr_t* base_of_stack_pointer() { return NULL; }
- void record_base_of_stack_pointer() {}
-
- // These routines are only used on cpu architectures that
- // have separate register stacks (Itanium).
- static bool register_stack_overflow() { return false; }
- static void enable_register_stack_guard() {}
- static void disable_register_stack_guard() {}
-