Don't scare users with a warning about preemption being off when it isn't

yet safe to have on by default.

sys/kern/kern_switch.c

@@ -537,6 +537,7 @@ maybe_preempt(struct thread *td)
 #endif
 }
 
+#if 0
 #ifndef PREEMPTION
 /* XXX: There should be a non-static version of this. */
 static void
@@ -549,6 +550,7 @@ static char preempt_warning[] =
 SYSINIT(preempt_warning, SI_SUB_COPYRIGHT, SI_ORDER_ANY, printf_caddr_t,
     preempt_warning)
 #endif
+#endif
 
 /************************************************************************
  * SYSTEM RUN QUEUE manipulations and tests				*