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

Since the previous commit, CBlockHeader/CBlock 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/mining_mainnet.py

@@ -66,7 +66,6 @@ class MiningMainnetTest(BitcoinTestFramework):
         block.vtx[0].nLockTime = 0
         block.vtx[0].vin[0].nSequence = SEQUENCE_FINAL
         block.hashMerkleRoot = block.calc_merkle_root()
-        block.rehash()
         block_hex = block.serialize(with_witness=False).hex()
         self.log.debug(block_hex)
         assert_equal(node.submitblock(block_hex), None)