chore(users/gs/owothia): remove unnecessary overrides, simplify

* chatter and its dependencies have been fixed in upstream
  haskellPackages, so we can get rid of the packageSet.nix expression:
  https://github.com/NixOS/nixpkgs/pull/116803

* Merged default.nix and pkg.nix into one expression and use
  callCabal2nix instead of emulating it with nested imports.

* Minor refactor of shell.nix and remove haskell-language-server
  until we've redone it or replaced it with the upstream one as now the
  GHC versions are out of sync: hls is built with 8.8.4, but
  haskellPackages uses 8.10.4

Change-Id: Ie75eaa93ba8bd79e749e2442fb28c855b8a15a1d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2661
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

users/glittershark/owothia/default.nix

@@ -1,6 +1,4 @@
-{ pkgs ? (import ../../../. {}).third_party
-, lib ? pkgs.lib
-, ...
-}:
+{ pkgs ? (import ../../../. {}).third_party, ... }:
 
-(import ./packageSet.nix {}).callPackage (import ./pkg.nix { inherit pkgs; }) {}
+pkgs.haskellPackages.callCabal2nix "owothia"
+  (pkgs.gitignoreSource ./.) { }