Remove unused using directives from API and client files

Cleaned up unnecessary System.Net and System.Net.Http.Json usings across multiple files, including controllers and API client classes, to reduce dependencies and improve code clarity.

DbFirst.BlazorWebApp/Services/CatalogApiClient.cs

@@ -1,5 +1,3 @@
-using System.Net;
-using System.Net.Http.Json;
 using DbFirst.BlazorWebApp.Models;
 
 namespace DbFirst.BlazorWebApp.Services;