RemoteDomainConnectionService should not shut down the provided executor

The WFLY-3225 fix changed RemoteDomainConnectionService from creating its own executor to accepting an external one via its constructor. But its stop() code was not modified to remove the call to shutdown the executor. As a result the service is shutting down an injected executor prematurely, leading to issues like https://bugzilla.redhat.com/show_bug.cgi?id=1112215.