chore(3p): use haskell-language-server from nixpkgs

Upstream haskellPackages has a newer haskell-language-server than we had
and it is always in sync with the default GHC version of that package
set which we incidentally use to build all haskell derivations in depot.

I hope this will not cause us more trouble in the future, but I've
gotten the impression that maralorn makes an effort to prevent
haskell-language-server from being broken in haskellPackages, so ideally
we'll never have to worry about hls again. If we do have to, we may need
to switch to easy-hls-nix.

Note that I haven't had the time yet to verify that the shells actually
work since it's kinda late now and the rebuilds are many.

Change-Id: I74c192d57355904cfa45bb76d70346792ba05af5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2662
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

users/glittershark/xanthous/shell.nix

@@ -11,6 +11,6 @@
     ghc-prof-flamegraph
     hp2pretty
     hlint
-    pkgs.haskell-language-server.ghc884
+    haskell-language-server
   ];
 }