Get rid of world files

[ezyang]

They were added in 9efcc5b

They are undocumented.

There are no tests.

Nix-local build will probably simulate worlds using a different mechanism.

Let's get rid of it.

CC @dcoutts

[23Skidoo]

I never use this feature, but the basic idea is that it lets you upgrade to a new GHC version and then run cabal install world to reinstall all packages that you previously installed manually. So the question is what is the proposed new mechanism that nix-local-build will use to achieve the same goal.

[ezyang]

Well, assuming you have some list of extra-packages as your world, if you just change the compiler it will autorebuild everything. See #3737

[dcoutts]

Yes I think we can abandon the world stuff in new-build at least, and replace it with some other mechanism to conveniently manage the global environment. I wouldn't bother to get rid of it from the legacy code though.

[ezyang]

Issue subsumed by PR #3749.