Remove vestigial code for old versions

[braingram]

After #232 we can go through the codebase and remove code that is no longer reachable.

I think some of these (the asdf-transform-schemas ones) go beyond the deps changed in the linked PR (another reason to have this be a separate PR):
https://github.com/search?q=repo%3Aastropy%2Fasdf-astropy%20minversion&type=code