Revert block hash object

[lightclient]

Reverts a change from #326 which turns block hashes into an object. I misunderstood this change and CI did not catch the issue in the tests.

[jaypaik]

Just stumbled upon this while looking into https://eips.ethereum.org/EIPS/eip-1898.

I thought I'd add that the EIP explicitly allows for objects that look like: { "blockHash": "0xd4e56740f876aef8c010b86a40d5f56745a118d0906a34e69aec8c0db1cb8fa3", "requireCanonical": false }.

And here's where Geth supports this: https://github.com/ethereum/go-ethereum/blob/452a12aa7903209713ccdc54af65a1a31e73190d/rpc/types.go#L147-L156.

What the EIP does not explicitly specify (at least from my reading of it) is the ability to also pass in block hashes without a wrapper object like above, but it seems that the Geth team allowed this anyway here: https://github.com/ethereum/go-ethereum/blob/452a12aa7903209713ccdc54af65a1a31e73190d/rpc/types.go#L184-L192.