remove unnecessary usings from solution, fixes in metrics

MareSynchronosServer/MareSynchronosStaticFilesServer/Startup.cs

@@ -9,7 +9,6 @@ using Microsoft.AspNetCore.Hosting;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
-using Microsoft.Extensions.FileProviders;
 using Microsoft.Extensions.Logging;
 using Prometheus;
 using System;