test: remove unnecessary block rehashing prior to solving

Solving a block involves continously rehashing it, i.e. any extra
calls to rehash it before are not necessary and can be dropped.

test/functional/feature_block.py

@@ -612,7 +612,6 @@ class FullBlockTest(BitcoinTestFramework):
         b45.nBits = 0x207fffff
         b45.vtx.append(non_coinbase)
         b45.hashMerkleRoot = b45.calc_merkle_root()
-        b45.calc_sha256()
         b45.solve()
         self.block_heights[b45.sha256] = self.block_heights[self.tip.sha256] + 1
         self.tip = b45