Fix block struct according to the spec

* `totalDifficulty` field removed as it was removed from the eth
  spec in ethereum/execution-apis#570
* `difficulty` field is not required, making it optional

Author: jinchui-crypto

packages/ethereum/src/Network/Ethereum/Api/Types.hs

@@ -277,10 +277,8 @@ data BlockT tx = Block
     -- ^ DATA, 32 Bytes - the root of the receipts trie of the block.
     , blockMiner            :: !Address
     -- ^ DATA, 20 Bytes - the address of the beneficiary to whom the mining rewards were given.
-    , blockDifficulty       :: !Quantity
+    , blockDifficulty       :: !(Maybe Quantity)
     -- ^ QUANTITY - integer of the difficulty for this block.
-    , blockTotalDifficulty  :: !(Maybe Quantity)
-    -- ^ QUANTITY - integer of the total difficulty of the chain until this block.
     , blockExtraData        :: !HexString
     -- ^ DATA - the "extra data" field of this block.
     , blockSize             :: !Quantity