Revert "Add gemini-2.5-flash to Firebase AI sample app"

This reverts commit 4ffb31e.

Author: dlarocque

ai/ai-react-app/src/services/firebaseAIService.ts

@@ -19,7 +19,6 @@ export const AVAILABLE_GENERATIVE_MODELS = [
   "gemini-2.0-flash",
   "gemini-2.0-flash-lite",
   "gemini-2.0-flash-exp",
-  "gemini-2.5-flash"
 ];
 export const AVAILABLE_IMAGEN_MODELS = ["imagen-3.0-generate-002"];