Make sure replication will remove remote branches that are absent locally

See
https://gerrit.googlesource.com/plugins/replication/+doc/master/src/main/resources/Documentation/config.md#file-1
for more details about the "remote.NAME.mirror" option.

Bug: Issue 12104
Change-Id: Ibb0210cb9367a1c33260cbfde520714b97161736
diff --git a/setup_local_env/configs/replication.config b/setup_local_env/configs/replication.config
index cf7f66a..ece1b3e 100644
--- a/setup_local_env/configs/replication.config
+++ b/setup_local_env/configs/replication.config
@@ -1,6 +1,7 @@
 [remote "Replication"]
     $REPLICATION_URL
     push = +refs/*:refs/*
+    mirror = true
     timeout = 600
     rescheduleDelay = 15
     replicationDelay = $REPLICATION_DELAY_SEC