validation: remove nchaintx from assumeutxo metadata

This value is no longer used and is instead specified statically
in chainparams. This change means that previously generated
snapshots will no longer be usable.

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(