Remove broken top-level shell.nix
Remove broken top-level shell.nix

There is a maintained shell.nix file in the hadrian submodule. This
has long since been broken anyway.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: thomie, carter

GHC Trac Issues: #15045

Differential Revision: https://phabricator.haskell.org/D4599

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc