Merge bitcoin/bitcoin#21681: validation: fix ActivateSnapshot to use hardcoded nChainTx

91d93aac4e validation: remove nchaintx from assumeutxo metadata (James O'Beirne)
931684b24a validation: fix ActivateSnapshot to use hardcoded nChainTx (James O'Beirne)

Pull request description:

  This fixes an oversight from the move of nChainTx from the user-supplied
  snapshot metadata into the hardcoded assumeutxo chainparams.

  Since the nChainTx is now unused in the metadata, it should be removed
  in a future commit.

  See: https://github.com/bitcoin/bitcoin/pull/19806#discussion_r612165410

ACKs for top commit:
  Sjors:
    utACK 91d93aac4e
  ryanofsky:
    Code review ACK 91d93aac4e. No change to previous commit, just new commit removing now unused utxo snapshot field and updating tests.

Tree-SHA512: 445bdd738faf007451f40bbcf360dd1fb4675e17a4c96546e6818c12e33dd336dadd95cf8d4b5f8df1d6ccfbc4bf5496864bb5528e416cea894857b6b732140c

test/functional/rpc_dumptxoutset.py

@@ -41,7 +41,7 @@ class DumptxoutsetTest(BitcoinTestFramework):
             digest = hashlib.sha256(f.read()).hexdigest()
             # UTXO snapshot hash should be deterministic based on mocked time.
             assert_equal(
-                digest, 'be032e5f248264ba08e11099ac09dbd001f6f87ffc68bf0f87043d8146d50664')
+                digest, '7ae82c986fa5445678d2a21453bb1c86d39e47af13da137640c2b1cf8093691c')
 
         # Specifying a path to an existing file will fail.
         assert_raises_rpc_error(