[C++] clean up unused stuff from jsonserializer/CMakeLists.txt
[C++] clean up unused stuff from jsonserializer/CMakeLists.txt

Change-Id: I4bd523f4e78115434cd5d8a7f5d55c2b5e6b89ca

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