Avoid calling TRANSMITTERS.keys to save some gas

[krebernisak]

@archseer suggested this part could possibly be refactored to save some gas.

Consider this snippet from contracts/ocr2/src/contract.rs:

    // validate new config
    require!(f != 0, InvalidInput);
    require!(signers_len <= MAX_ORACLES, TooManySigners);
    require!(transmitters.len() == signers.len(), InvalidInput);
    require!(3 * (usize::from(f)) < signers_len, InvalidInput);
    require!(onchain_config.is_empty(), InvalidInput);
    require!(!offchain_config.is_empty(), InvalidInput);

    let (_total, mut response) = pay_oracles(&mut deps, &config, response)?;
    // TODO: pay_oracles already loads all the transmitters, avoid calling TRANSMITTERS.keys

    // Clear out oracles
    let keys: Vec<_> = TRANSMITTERS
        .keys(deps.storage, None, None, Order::Ascending)
        .collect();
    for key in keys {
        TRANSMITTERS.remove(deps.storage, &to_addr(key));
    }
    let keys: Vec<_> = SIGNERS
        .keys(deps.storage, None, None, Order::Ascending)
        .collect();
    for key in keys {
        SIGNERS.remove(deps.storage, &key);
    }