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/lint.yml

@@ -77,17 +77,3 @@ jobs:
         if: steps.changed-files.outputs.files == 'true'
         run: nix develop --fallback --command -- prettier --no-error-on-unmatched-pattern
           --ignore-unknown --check **/*.{ts,js,md,yaml,yml,sass,css,scss,html}
-
-  proto-lint:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-      - uses: nixbuild/nix-quick-install-action@2c9db80fb984ceb1bcaa77cdda3fdf8cfba92035 # v34
-      - uses: nix-community/cache-nix-action@135667ec418502fa5a3598af6fb9eb733888ce6a # v6.1.3
-        with:
-          primary-key: nix-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('**/*.nix',
-            '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ runner.arch }}
-
-      - name: Buf lint
-        run: nix develop --fallback --command -- buf lint proto