test: remove bare CTransaction .rehash()/.calc_sha256() calls

Since the previous commit, CTransaction object calls to the
methods `.rehash()` and `.calc_sha256()` are effectively no-ops
if the returned value is not used, so we can just remove them.

test/functional/feature_nulldummy.py

@@ -45,7 +45,6 @@ def invalidate_nulldummy_tx(tx):
     to be non-NULLDUMMY compliant by replacing the dummy with OP_TRUE"""
     assert_equal(tx.vin[0].scriptSig[0], OP_0)
     tx.vin[0].scriptSig = bytes([OP_TRUE]) + tx.vin[0].scriptSig[1:]
-    tx.rehash()
 
 
 class NULLDUMMYTest(BitcoinTestFramework):