validation: fix ActivateSnapshot to use hardcoded nChainTx

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.

src/node/utxo_snapshot.h

@@ -24,6 +24,8 @@ public:
 
     //! Necessary to "fake" the base nChainTx so that we can estimate progress during
     //! initial block download for the assumeutxo chainstate.
+    //!
+    //! TODO: this is unused and should be removed.
     unsigned int m_nchaintx = 0;
 
     SnapshotMetadata() { }