Tidy some TODOs (#1721)

## Issue Addressed

- Resolves #1705

## Proposed Changes

Cleans up some of my TODOs in the code base.

- Adds link to issue in this repo for BLST `unsafe` block.
- Confirms that the `nextaccount` field *is* required on an EIP-2386 wallet.
    - Reference: https://github.com/mcdee/EIPs/blob/master/EIPS/eip-2386.md#json-schema
- Removes TODO about Zeroize on bip39 that was resolved in #1701 
- Removes a TODO about an early randao reveal since we use the slot clock to generate the reveal: c4bd9c86e6/validator_client/src/block_service.rs (L212-L220)

## Additional Info

NA

crypto/eth2_wallet/src/wallet.rs

@@ -66,7 +66,6 @@ impl<'a> WalletBuilder<'a> {
         password: &'a [u8],
         name: String,
     ) -> Result<Self, Error> {
-        // TODO: `bip39` does not use zeroize. Perhaps we should make a PR upstream?
         let seed = Bip39Seed::new(mnemonic, "");
 
         Self::from_seed_bytes(seed.as_bytes(), password, name)