Remove AI assistant references from VS Code extension

[JamesNK]

After removing the AI assistant feature from the dashboard (#18726), the VS Code extension still has leftover references that should be cleaned up:

1. extension/src/debugger/languages/dotnet.ts — Injects ASPIRE_DASHBOARD_AI_DISABLED=true env var for every AppHost launch. The dashboard no longer recognizes this variable.

2. extension/src/debugger/languages/maui.ts — Lists ASPIRE_DASHBOARD_AI_DISABLED in isKnownMauiApplicationEnvironmentVariable.

3. extension/src/dcp/DashboardTelemetryPassthrough.ts — Allowlists 5 Aspire.Dashboard.AIAssistant.* property keys in ALLOWED_DASHBOARD_PROPERTY_KEYS and ALLOWED_STRING_ARRAY_PROPERTY_KEYS.

4. extension/src/test/dashboardTelemetryPassthrough.test.ts — Tests exercise the removed AI property keys.

These are harmless (they emit a now-ignored env var and allowlist telemetry keys that will never arrive), but should be removed to keep the extension in sync with the dashboard.

[JamesNK]

Might want to wait before removing these. Considering the scenario of a new extension with an old app host.