chore: remove legacy_preupgrade feature

[ielashi]

The Bitcoin mainnet canister has now been updated to the latest release and this feature isn't needed anymore.

[github-actions]

canbench 🏋 (dir: .)

No significant performance changes detected ✅

./canbench_results.yml is up to date ✅

~/work/bitcoin-canister/bitcoin-canister/scripts ~/work/bitcoin-canister/bitcoin-canister
~/work/bitcoin-canister/bitcoin-canister

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Benchmark: insert_300_blocks
  total:
    instructions: 561.76 M (-0.00%) (change within noise threshold)
    heap_increase: 6 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: get_metrics
  total:
    instructions: 87.00 M (-0.00%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers
  total:
    instructions: 3.90 B (0.00%) (change within noise threshold)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 13.90 B (0.02%) (change within noise threshold)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: pre_upgrade_with_many_unstable_blocks
  total:
    instructions: 5.82 B (0.21%) (change within noise threshold)
    heap_increase: 4097 pages (no change)
    stable_memory_increase: 1792 pages (no change)

  serialize_blocktree (scope):
    instructions: 2.39 B (0.45%) (change within noise threshold)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_flatten (scope):
    instructions: 201.60 K (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_serialize_seq (scope):
    instructions: 2.39 B (0.45%) (change within noise threshold)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

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