Remove legacy group membership fields from backend

[megrogan]

No description provided.

[github-actions]

canbench 🏋 (dir: .)

./canbench_results.yml is up to date ✅

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Benchmark: push_simple_text_messages
  total:
    instructions: 205.78 M (-1.19%) (change within noise threshold)
    heap_increase: 13 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: add_reactions
  total:
    instructions: 1.22 B (-1.04%) (change within noise threshold)
    heap_increase: 4 pages (no change)
    stable_memory_increase: 0 pages (no change)

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