hscontrol: remove the proto, gRPC and grpc-gateway stack

With the v1 API served by Huma and the CLI on the HTTP client, nothing
uses the gRPC service or its generated code. Delete proto/, the generated
gen/go and gen/openapiv2, grpcv1.go, the buf config, the proto .Proto()
helpers and gRPC config, and the proto build tooling from the flake and
CI.

.github/workflows/check-generated.yml

@@ -28,8 +28,6 @@ jobs:
               - '*.nix'
               - 'go.*'
               - '**/*.go'
-              - '**/*.proto'
-              - 'buf.gen.yaml'
               - 'tools/**'
       - uses: nixbuild/nix-quick-install-action@2c9db80fb984ceb1bcaa77cdda3fdf8cfba92035 # v34
         if: steps.changed-files.outputs.files == 'true'