test: introduce and use CTransaction .wtxid_int property

This commits removes the `.calc_sha256` method from the CTransaction
and introduces a property `.wtxid_int` property as replacement.

test/functional/p2p_tx_privacy.py

@@ -71,7 +71,7 @@ class TxPrivacyTest(BitcoinTestFramework):
 
         # Spy should only get an inv for the second transaction as the first
         # one was received pre-verack with the spy
-        spy.wait_for_inv_match(CInv(MSG_WTX, tx2.calc_sha256(True)))
+        spy.wait_for_inv_match(CInv(MSG_WTX, tx2.wtxid_int))
 
 if __name__ == '__main__':
     TxPrivacyTest(__file__).main()