revert a previous change that caused existing system collections on a slave to be truncated instead of being deleted (#8443)

truncating instead of deleting introduced the possibility of the collection's indexes continuing to exist with different ids on the slave than on the master, leading to potential follow-up problems

arangod/Replication/DatabaseInitialSyncer.cpp

@@ -1128,9 +1128,10 @@ Result DatabaseInitialSyncer::handleCollection(VPackSlice const& parameters,
         if (col != nullptr) {
           bool truncate = false;
 
-          if (col->system()) {
-            // better not throw away system collections. otherwise they may be dropped
-            // and this can be a problem if the server crashes before they are recreated.
+          if (col->name() == TRI_COL_NAME_USERS) {
+            // better not throw away the _users collection. otherwise it is gone
+            // and this may be a problem if the
+            // server crashes in-between.
             truncate = true;
           }