examples/lorawan: don't unblock IDLE mode

This is already unblocked and will trigger an assertion.
The code is still broken as other modes might be unblocked too,
but at least it is just as broken as it was before RIOT-OS#17895

examples/lorawan/main.c

@@ -154,7 +154,7 @@ int main(void)
      * in general provides RAM retention after wake-up.
      */
 #if IS_USED(MODULE_PM_LAYERED)
-    for (unsigned i = 1; i < PM_NUM_MODES; ++i) {
+    for (unsigned i = 1; i < PM_NUM_MODES - 1; ++i) {
         pm_unblock(i);
     }
 #endif