sched: remove slew handler in finalization

This allows repeated calls of SCH_Initialise() and SCH_Finalise().

sched.c

@@ -147,6 +147,8 @@ void
 SCH_Finalise(void) {
   ARR_DestroyInstance(file_handlers);
 
+  LCL_RemoveParameterChangeHandler(handle_slew, NULL);
+
   initialised = 0;
 }