modules: use assertions module from nixpkgs

It is essentially identical to our `warnings` module.

modules/default.nix

@@ -15,6 +15,5 @@
     ./opts.nix
     ./output.nix
     ./plugins.nix
-    ./warnings.nix
   ];
 }

modules/misc/default.nix

@@ -8,6 +8,7 @@ in
   imports = [
     ./nixpkgs.nix
     ./nixvim-info.nix
+    (nixosModules + "/misc/assertions.nix")
     (nixosModules + "/misc/meta.nix")
   ];
 }

modules/warnings.nix

@@ -1,16 +0,0 @@
-{ lib, ... }:
-with lib;
-{
-  options = {
-    warnings = mkOption {
-      type = types.listOf types.str;
-      visible = false;
-      default = [ ];
-    };
-    assertions = mkOption {
-      type = types.listOf types.attrs; # Not sure what the correct type is here
-      visible = false;
-      default = [ ];
-    };
-  };
-}