Boost: Clean up Viewport classes and configuration

Originally posted by @pyronaur.

Configuration is setting up multiple viewport configuration variables and it's not clear how they're used or what they do::

https://github.com/Automattic/jetpack/blob/29ccc2fc37522edf221a2e38363fae87516b7345/projects/plugins/boost/app/class-jetpack-boost.php#L417-L428

Half of the code in [`class Viewport`](https://github.com/Automattic/jetpack/blob/master/projects/plugins/boost/app/lib/class-viewport.php) seems to be unused, and when we use viewports, we define them without actually using the config anyway:

https://github.com/Automattic/jetpack/blob/29ccc2fc37522edf221a2e38363fae87516b7345/projects/plugins/boost/app/modules/critical-css/class-critical-css.php#L36-L52

I think we need to re-think how we deal with options and customization. Making everything hot-swappable makes the configuration also obscure and difficult to work with, so we end up just localizing the values to the code we're working with anyway, ending up in more confusion everywhere else.

Start by comparing the configuration in projects/plugins/boost/app/class-jetpack-boost.php, the Viewport implementation in projects/plugins/boost/app/lib/class-viewport.php, and its use in projects/plugins/boost/app/modules/critical-css/class-critical-css.php. Trace which viewport options are read or unused, then clarify the intended configuration and customization model before defining what cleanup is complete.

Written by the indexing model from the issue text.