Remove redundant Nitro development cleanup after upstream release

Context

Workflow #2908 added two kinds of cleanup:

• a buildEnd hook that closes the temporary Nitro development instance;
• disposal of Workflow-owned esbuild contexts.

nitrojs/nitro #4362 fixes Nitro's environment-runner cleanup and is merged on Nitro main, but is not in the Nitro v3 release used by #2908.

A/B verification in Workflow #2923 and Workflow #2930 confirmed that Workflow v4 and v5 production builds exit after disposing their build contexts without calling the process-global esbuild.stop(). Workflow therefore does not retain a global esbuild shutdown hook.

Follow-up

Once Workflow can require a Nitro release containing nitrojs/nitro#4362:

• Remove the devNitro tracking and development buildEnd / nitro.close() workaround from packages/nitro/src/vite.ts.
• Remove or update the corresponding development lifecycle test.
• Keep the Workflow-owned build-context disposal.
• Verify React Router v7 and v8 development and production builds still work and exit.

The temporary React Router manifest bridge documented in #2930 is separate. Remove it only after Nitro provides equivalent React Router client-output handling while preserving deployment-preset output directories.

Start in packages/nitro/src/vite.ts and inspect the corresponding development lifecycle test after confirming the required Nitro release includes nitrojs/nitro#4362. Remove the obsolete development cleanup workaround while retaining Workflow-owned build-context disposal, then verify React Router v7 and v8 development and production builds still work and exit.