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_sendheaders.py

@@ -150,7 +150,6 @@ class BaseNode(P2PInterface):
         if len(message.headers):
             self.block_announced = True
             for x in message.headers:
-                x.calc_sha256()
                 # append because headers may be announced over multiple messages.
                 self.recent_headers_announced.append(x.sha256)
             self.last_blockhash_announced = message.headers[-1].sha256