Strip out the Posts implementation

For prototyping we used the [Posts](https://developer.wordpress.org/rest-api/reference/posts/) endpoint, however we decided not to fully implement it for v0.1. Since [we are aiming for a polished and production quality code in v0.1](https://github.com/Automattic/wordpress-rs/issues/11), we'll strip out the existing implementation to later bring it back when we are working on the Posts endpoint.