Drop support for Overlay and Embedding from PileUpEventLoopMgr and clean-up whitespace (ATLASSIM-5648)

Once !50572 (merged) is merged then the the PileUpEventLoopMgr will only be used in the pile-up digitization (pile-up presampling) workflow. This allows the code to be greatly simplified, which will in turn make it easier to refactor it to support reading HITS files containing xAOD::EventInfo rather than old-style EventInfo.
This merge request is mostly concerned with removing support for the Data/MC Overlay workflows and for the embedding workflow. The first two are run in production using the (thread-safe) DoubleEventSelector + the standard event loop managers and the same approach would most likely work best for emedding also.
I have made some whitespace/syntax clean-up in this merge request, but I would prefer to follow up on other changes of this nature in a separate MR, as the simplification of the class is holding up other developments.