[AI Improvement] [Task] Delete orphaned mockDiscoveryServer in functions runtime discovery

[joehan]

Resolves Buganizer b/563464527

Proposed Improvement

Deletes the orphaned and unused src/deploy/functions/runtimes/discovery/mockDiscoveryServer.ts.

• mockDiscoveryServer.ts was originally added in commit 0b0459bfde as an in-process Express mock server for discovery endpoints.
• In commit bd67ccbcdd, tests were refactored to use nock for HTTP interception, removing all references and imports to mockDiscoveryServer.ts.
• Repository-wide audit confirmed zero imports or references across src/, scripts/, and tests.
• Removing this dead file cleans up unused legacy code and reduces codebase clutter.

Verification

• Ran discovery unit tests: mocha 'src/deploy/functions/runtimes/discovery/*.spec.ts' — all 157 tests passed cleanly.
• Ran full TypeScript compilation and asset build: npm run build — built successfully with zero errors.

[gemini-code-assist]

Code Review

This pull request removes an Express.js server implementation that served a backend YAML configuration file on port 8080 (or a port specified by the ADMIN_PORT environment variable). I have no feedback to provide.