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_template_verification.py

@@ -158,7 +158,6 @@ class MiningTemplateVerificationTest(BitcoinTestFramework):
         # Ensure that it doesn't meet the target by coincidence
         while block.sha256 <= target:
             block.nNonce += 1
-            block.rehash()
         self.log.debug("Found a nonce")
 
         self.log.info("A block template doesn't need PoW")