`void cpu_switch_context_exit(void)` assigns `sched_active_thread` just before
calling `sched_run()`. This is unneeded, as `sched_run()` will updated that
anyway. Also generally speaking, changing internal scheduler data from outside
the scheduler is a risky thing to do.
- removed ISR_STACKSIZE define where unused (set to 0)
- removed thread_arch_isr_stack_usage(), thread_arch_isr_stack_start(),
and/or thread_arch_isr_stack_pointer() where not implemented