Remove unused using directives from Program.cs

Removed the following using directives from `Program.cs`:
- `Azure.Identity`
- `Azure.Search.Documents.Indexes`
- `Microsoft.EntityFrameworkCore`
- `OpenAI`
- `OpenAI.Chat`
- `OpenAI.Embeddings`
- `Products.Endpoints`
- `Products.Memory`

These namespaces are no longer needed due to refactoring,
removal of dependencies, or changes in the application's
architecture.

src/Products/Program.cs

@@ -1,6 +1,4 @@
-using Azure.Identity;
 using Azure.Search.Documents.Indexes;
-using Microsoft.EntityFrameworkCore;
 using OpenAI;
 using OpenAI.Chat;
 using OpenAI.Embeddings;