[staking-example] use chain-registry/utils and cleanup use of assets/chains

[pyramation]

The staking example hard-codes the usage of assets in at least two files:

https://github.com/cosmology-tech/create-cosmos-app/blob/ec225356f212efebfb0c794bac77ab2c6bf883eb/examples/stake-tokens/config/defaults.ts

create-cosmos-app/examples/stake-tokens/pages/_app.tsx

Line 10 in ec22535

import { chains, assets } from 'chain-registry';

let's at least simplify it to a single file so it can be imported, and also use chain-registry/utils for methods accessing chain-registry info

[HebeHe1986]

We should also look into using chain-registry/utils in other examples, but let’s start with this one and clean things up a bit