Update CollectionMerger to use SG::ReadHandleKeyArray member variables

This commit updates the ISF::CollectionMerger from using std::vector<std::string> to hold the names of input collections then building vectors of SG::ReadHandleKeys during initialize to using SG::ReadHandleKeyArray member variables directly. This simplifies the class considerably and will ensure that the Algorithm is scheduled correctly in AthenaMT jobs.

Also took the opportunity to clear up some other unnecessary code.