Rely entirely on proxy code to polyfill

[dlongley]

The code here:

https://github.com/credential-handler/credential-handler-polyfill/blob/v4.0.0/lib/index.js#L83-L90

Might be able to be removed such that we rely entirely on the subsequent proxy-based polyfill and Object.defineProperty approach. This approach seems to be more amenable in the face of other extensions that erroneously mark .get() and / or .store() as read-only.