[config] remove CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT (#10915)

This commit removes the `CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT`
configuration option.

The related behavior is now always enabled, meaning a device will
always send a Link Request when the advertisement timeout is reached
and the neighbor age threshold is met for a neighboring router.

src/core/config/mle.h

@@ -204,15 +204,6 @@
 #define OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_DELAY_TO_RESET_BACKOFF_INTERVAL 20000
 #endif
 
-/**
- * @def OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT
- *
- * Define to 1 to send an MLE Link Request when MAX_NEIGHBOR_AGE is reached for a neighboring router.
- */
-#ifndef OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT
-#define OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT 1
-#endif
-
 /**
  * @def OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN
  *

src/core/config/openthread-core-config-check.h

@@ -690,4 +690,8 @@
 #error "OPENTHREAD_CONFIG_BORDER_ROUTING_STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE is removed"
 #endif
 
+#ifdef OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT
+#error "OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT is removed, behavior is always applied"
+#endif
+
 #endif // OPENTHREAD_CORE_CONFIG_CHECK_H_

src/core/thread/mle_router.cpp

@@ -1653,14 +1653,12 @@ void MleRouter::HandleTimeTick(void)
 
         if (router.IsStateValid() && (age >= kMaxNeighborAge))
         {
-#if OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT
             if (age < kMaxNeighborAge + kMaxTxCount * kUnicastRetxDelay)
             {
                 LogInfo("No Adv from router 0x%04x - sending Link Request", router.GetRloc16());
                 IgnoreError(SendLinkRequest(&router));
             }
             else
-#endif
             {
                 LogInfo("Router 0x%04x timeout expired", router.GetRloc16());
                 RemoveNeighbor(router);