MdtRawDataMonitoring: Memory-leak fix and clean-up

This MR should fix the memory leaks that I bumped into in the context of ATLASDQ-838. I cleaned up by-hand heap allocations in MdtRawDataMonAlg::initialize(). From a quick look, I don't think we really need to keep these histograms in memory longer than the initialization anyways. Let me cc @mbigliet in case she notices anything weird.