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/rpc_blockchain.py

@@ -480,7 +480,6 @@ class BlockchainTest(BitcoinTestFramework):
         assert_is_hex_string(header_hex)
 
         header = from_hex(CBlockHeader(), header_hex)
-        header.calc_sha256()
         assert_equal(header.hash, besthash)
 
         assert 'previousblockhash' not in node.getblockheader(node.getblockhash(0))