[C++] cleanup ContentWithDecayTime.h
[C++] cleanup ContentWithDecayTime.h

* remove unused includes
* remove unused typedef

Change-Id: I33d58d7fae6f5c6a0e99ef9f59c7b547a9a385ad

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr