bitcoind: set onionPort in bitcoind module

This removes the module-level dependency from onion-services to
bitcoind.
Due to the `or false` fallback, there's no dependency added in
the reverse direction.

In particular, this allows us to not add a dependency on liquidd in
the following commit.

modules/onion-services.nix

@@ -118,10 +118,6 @@ in {
           externalPort = 80;
         };
       };
-
-      # When the bitcoind onion service is enabled, add an onion-tagged socket
-      # to distinguish local connections from Tor connections
-      services.bitcoind.onionPort = mkIf (cfg.bitcoind.enable or false) 8334;
     }
   ];
 }