sched: fix handling of signals after finalization

Don't require the scheduler to be initialized in SCH_QuitProgram().
This fixes a crash when a signal is received between scheduler
finalization and chronyd exit.

sched.c

@@ -719,7 +719,6 @@ SCH_MainLoop(void)
 void
 SCH_QuitProgram(void)
 {
-  assert(initialised);
   need_to_exit = 1;
 }