stake-pool-js: Fix build for downstream users

[joncinque]

Problem

There's an issue for downstream users of the build because there's a confusion between the locally used buffer-layout and the one used by borsh, which manifests as:

*/node_modules/buffer-layout/lib/Layout.js:1158
      throw new TypeError('fields must be array of Layout instances');
      ^

TypeError: fields must be array of Layout instances
    at new Structure (/Users/user/frontproject/earn-web3-node/node_modules/buffer-layout/lib/Layout.js:1158:13)
    at exports.struct (/Users/user/frontproject/earn-web3-node/node_modules/buffer-layout/lib/Layout.js:2688:57)
    at Object.<anonymous> (/Users/user/frontproject/earn-web3-node/node_modules/@solana/spl-stake-pool/dist/index.cjs.js:855:31)

Solution

Remove the borsh dependency and just use what's needed from buffer-layout directly.