nix: remove uneeded slocat

slocat was introduced to test pipeline mode (see https://github.com/PostgREST/postgrest/pull/2682),
but pipeline mode was not merged https://github.com/PostgREST/postgrest/pull/2707.

So it's really not needed on the loadtests.

default.nix

@@ -44,7 +44,6 @@ let
       allOverlays.checked-shell-script
       allOverlays.gitignore
       (allOverlays.haskell-packages { inherit compiler; })
-      allOverlays.slocat
     ];
 
   # Evaluated expression of the Nixpkgs repository.