Simplify muon trigger step dictionary

Savanna Shaw requested to merge sshaw/athena:muon-chaindef into master

Cleanup of the muon trigger chain step dictionary (ATR-24222):

  • Flatten the structure of the steps, since the [[L2 steps],[EF steps]] structure isn't actually needed
  • Remove unused chain step definitions
  • Consolidate the definitions with similar steps (mostly by moving the setup of the muFast/muComb steps to the getmuFast/getmuComb functions)

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