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_csv_activation.py

@@ -177,7 +177,6 @@ class BIP68_112_113Test(BitcoinTestFramework):
         block.nVersion = 4
         block.vtx.extend(txs)
         block.hashMerkleRoot = block.calc_merkle_root()
-        block.rehash()
         block.solve()
         return block