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/p2p_segwit.py

@@ -230,7 +230,6 @@ class SegWitTest(BitcoinTestFramework):
         height = self.nodes[0].getblockcount() + 1
         block_time = self.nodes[0].getblockheader(tip)["mediantime"] + 1
         block = create_block(int(tip, 16), create_coinbase(height), block_time)
-        block.rehash()
         return block
 
     def update_witness_block_with_transactions(self, block, tx_list, nonce=0):