locator: abstract_replication_strategy: Comment out an unused member

_keyspace back reference is not used at the moment.
Not deleting since we may need it when we add more code (quite soon).

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>

locator/abstract_replication_strategy.hh

@@ -23,7 +23,8 @@ using token = dht::token;
 class abstract_replication_strategy {
 protected:
     sstring _ks_name;
-    keyspace* _keyspace = nullptr;
+    // TODO: Do we need this member at all?
+    //keyspace* _keyspace = nullptr;
     std::map<sstring, sstring> _config_options;
     token_metadata& _token_metadata;
     snitch_ptr& _snitch;