Remove legacy code from code base

[mpickering]

Cabal and cabal-install have a large amount of legacy code.

Much legacy code for supporting different Haskell compilers, which is surely bitrotted at this point (untested on CI)

• [ ] Support for UHC
• [ ] Support for GHCJS
• [ ] Support for Haskell Suite

v1-commands have been deprecated for a long while

• [ ] Remove v1-commands

nix support is also legacy and unused to my knowledge

• [ ] Remove nix support

Please contribute to this issue by listing any other legacy parts of the code base which should be removed.

The cost of these code paths is real, they must also be updated when performing refactoring or other improvements (see #10256 for example).