cpu/msp430_common: Cleanup

`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.

cpu/msp430_common/cpu.c

@@ -55,7 +55,6 @@ void *thread_isr_stack_start(void)
 
 NORETURN void cpu_switch_context_exit(void)
 {
-    sched_active_thread = sched_threads[0];
     sched_run();
 
     __restore_context();