haskellPackages.callHackage: updating all-cabal-hashes do not invalidate callHackage

Packages built with `haskellPackages.callHackage` won't be rebuilt when
updating `all-cabal-hashes`.

The removed comment was keeping a reference to the `cabal2nix` call,
which itself depends on `all-cabal-hashes`, in order to keep this file
during a garbage collection.

The tradeoff is between:

- The current behavior: a mass rebuild, any change of `all-cabal-hashes`
  triggers a rebuild of all the packages built with `callHackage` and
  packages which depend on them. This can take hours, and may happen
  after a "small" unrelated change (i.e. an user is bumping
  `all-cabal-hashes` in order to use a new package from hackage). It
  also have global impacts in a project (long rebuild in CI, new entries
  in cache, developers need to fetch the new entries, ...). In this
  context, `cabal2nix` entries are not garbage collected.
- The new behavior: No mass rebuild, but `cabal2nix` derivations need to
  be recomputed after a garbage collection. This is usually fast (a few
  seconds by call), linear with the number of calls and should not
  happen a lot (i.e. users are not garbage collecting everyday).

See https://github.com/NixOS/nixpkgs/issues/194751 for details.

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