TrigServices: Rename HltAsyncEventLoopMgr and drop legacy one

Now that the HltAsyncEventLoopMgr is used by default we can drop the old one and restore the original naming.

ℹ Note to reviewers: The are basically no code changes here. Just renaming of HltAsyncEventLoopMgr to HltEventLoopMgr.

cc @ssottoco @smh and relates to ATR-26285.