Remove lazy loading of "accessor" and "changeEvent" in Property

Related to what was discussed here: https://github.com/Shopify/batman/pull/598/files#r2296533

I removed the lazy-loading logic for `accessor` and `changeEvent` so that we don't need to call a function to obtain the values.

Perf. improvements are likely negligible, but these functions get hit quite a lot.

## Review

@kristianpd @hornairs

/cc @mattwdh @nciagra