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_nulldummy.py

@@ -130,7 +130,6 @@ class NULLDUMMYTest(BitcoinTestFramework):
         block.hashMerkleRoot = block.calc_merkle_root()
         if with_witness:
             add_witness_commitment(block)
-        block.rehash()
         block.solve()
         assert_equal(None if accept else NULLDUMMY_ERROR, node.submitblock(block.serialize().hex()))
         if accept: