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.

nix/overlays/default.nix

@@ -3,5 +3,4 @@
   checked-shell-script = import ./checked-shell-script;
   gitignore = import ./gitignore.nix;
   haskell-packages = import ./haskell-packages.nix;
-  slocat = import ./slocat.nix;
 }

nix/overlays/slocat.nix

@@ -1,13 +0,0 @@
-_: prev:
-{
-  slocat = prev.buildGoModule {
-    name = "slocat";
-    src = prev.fetchFromGitHub {
-      owner = "robx";
-      repo = "slocat";
-      rev = "52e7512c6029fd00483e41ccce260a3b4b9b3b64";
-      sha256 = "sha256-qn6luuh5wqREu3s8RfuMCP5PKdS2WdwPrujRYTpfzQ8=";
-    };
-    vendorHash = null;
-  };
-}