[mle] enable sending MLE Link Request on Adv timeout by default (#6713)

The `OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT` feature
has been incorporated into the Thread 1.2.1 draft specification.

src/core/config/mle.h

@@ -190,7 +190,7 @@
  *
  */
 #ifndef OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT
-#define OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT 0
+#define OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT 1
 #endif
 
 /**

tests/scripts/thread-cert/Cert_9_2_15_PendingPartition.py

@@ -125,6 +125,7 @@ class Cert_9_2_15_PendingPartition(thread_cert.TestCase):
 
         self.nodes[ROUTER2].reset()
         self._setUpRouter2()
+        self.simulator.go(100)
 
         self.nodes[COMMISSIONER].send_mgmt_pending_set(
             pending_timestamp=20,
@@ -133,7 +134,7 @@ class Cert_9_2_15_PendingPartition(thread_cert.TestCase):
             mesh_local='fd00:0db7::',
             panid=PANID_FINAL,
         )
-        self.simulator.go(101)
+        self.simulator.go(100)
 
         self.nodes[ROUTER2].start()
         self.simulator.go(5)

tests/toranj/openthread-core-toranj-config.h

@@ -339,14 +339,6 @@
  */
 #define OPENTHREAD_CONFIG_MLE_INFORM_PREVIOUS_PARENT_ON_REATTACH 1
 
-/**
- * @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.
- *
- */
-#define OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT 1
-
 /**
  * @def OPENTHREAD_CONFIG_CHANNEL_MANAGER_ENABLE
  *