Drop support for unsupported dependencies in v3

[tacman]

Drop support for unsupported dependencies in v3

[dbu]

i agree, we should cleanup up a lot here.

version 3.x did not diverge all that much from 2.x yet, so i don't expect bugs or broken things.

but i hope we can do some more cleanups that might lead to BC breaks - so using the 3.x branch might be bumpy when upgrading dependencies.

notably with issues like #1523 and many people with problems with paths, i would love to cleanup how we handle the paths (and fix the naming to be easier to understand). it should be simple to understand when its about storing the source image somewhere, when about storing a cached image and when its the url to load the image in a browser.
i also hope that we can ditch some of the code for supporting various storage solutions in favor of just league/flysystem for everything beyond the local filesystem. or maybe even hard depend on flysystem and use it for local files too.

[tacman]

Well, that'd be great. As I mentioned elsewhere, dropping OneUp would make documentation much easier.

I think my situation is probably pretty common -- I develop with a local filesystem, and occasionally with S3, then I deploy to production with S3. This works well once it's set up, but setting it up the first time was confusing for me.