[mle] do not clear address query cache when switching to the router role (#2529)

diff --git a/src/core/thread/mle_router.cpp b/src/core/thread/mle_router.cpp
index 8f57f1f..66a9f8e 100644
--- a/src/core/thread/mle_router.cpp
+++ b/src/core/thread/mle_router.cpp
@@ -441,7 +441,6 @@
     netif.GetIp6().SetForwardingEnabled(true);
     netif.GetIp6().GetMpl().SetTimerExpirations(kMplRouterDataMessageTimerExpirations);
     netif.GetMac().SetBeaconEnabled(true);
-    netif.GetAddressResolver().Clear();
 
     // clear router table
     for (int i = 0; i <= kMaxRouterId; i++)