Remove root devshell - templates are self-contained

The root flake only defines templates. Each template (templates/vanilla/
and templates/nextjs/) contains its own complete Nix configuration with
devshell, so root-level development environment is unnecessary.

flake.nix

@@ -11,7 +11,6 @@
     inputs.flake-parts.lib.mkFlake {inherit inputs;} {
       systems = import inputs.systems;
       imports = [
-        ./nix/modules/devshell.nix
         ./nix/modules/template.nix
       ];
     };