scripted-diff: Remove setup_clean_chain if default is not changed

-BEGIN VERIFY SCRIPT-
git grep -l "self.setup_clean_chain = False" test/functional/*.py | xargs sed -i "/self.setup_clean_chain = False/d";
-END VERIFY SCRIPT-

test/functional/p2p_tx_download.py

@@ -56,7 +56,6 @@ MAX_GETDATA_INBOUND_WAIT = GETDATA_TX_INTERVAL + INBOUND_PEER_TX_DELAY + TXID_RE
 
 class TxDownloadTest(BitcoinTestFramework):
     def set_test_params(self):
-        self.setup_clean_chain = False
         self.num_nodes = 2
 
     def test_tx_requests(self):