[C++] remove unused mockCallback and mockReplyCaller in SubscriptionTest
[C++] remove unused mockCallback and mockReplyCaller in SubscriptionTest

Change-Id: I6aeeef22a903b7a8768322c663afab2435495575

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr