Clean up sequence server routes

[cartermak]

Clean up the sequencing server's routes/seqjson.ts and routes/command-expansion.ts:

• Consolidate shared logic for invoking builders (lots of re-generating sortedSimulatedActivitiesWithCommands)
• Reduce duplicate simulated activity fetches
• Remove use of null for expansionResults to simplify processing those results
• Remove the long deprecated route /get-seqjson-for-seqid-and-simulation-dataset?

[cartermak]

Pairs well with a fix for #1648