[routing-manager] fix build and test failures (#7707)

`RemoveExternalRoute` -> `UnpublishExternalRoute`
diff --git a/src/core/border_router/routing_manager.cpp b/src/core/border_router/routing_manager.cpp
index 91b8a6d..0d99264 100644
--- a/src/core/border_router/routing_manager.cpp
+++ b/src/core/border_router/routing_manager.cpp
@@ -383,7 +383,7 @@
     {
         if (mIsAdvertisingLocalOnLinkPrefix)
         {
-            RemoveExternalRoute(mLocalOnLinkPrefix);
+            UnpublishExternalRoute(mLocalOnLinkPrefix);
             // TODO: consider deprecating/invalidating existing
             // on-link prefix
             mIsAdvertisingLocalOnLinkPrefix = false;