nix: Add deadnix to postgrest-lint and remove dead nix code

nix/tools/style.nix

@@ -2,6 +2,7 @@
 , black
 , buildToolbox
 , checkedShellScript
+, deadnix
 , git
 , hlint
 , hsie
@@ -65,10 +66,12 @@ let
         echo "Linting workflows..."
         ${actionlint}/bin/actionlint
 
+        echo "Scanning nix files for unused code..."
+        ${deadnix}/bin/deadnix
+
         echo "Checking consistency of import aliases in Haskell code..."
         ${hsie} check-aliases main src
 
-
         echo "Linting Haskell files..."
         # --vimgrep fixes a bug in ag: https://github.com/ggreer/the_silver_searcher/issues/753
         ${silver-searcher}/bin/ag -l --vimgrep -g '\.l?hs$' . \