Removed an obsolete using

Author: ckadluba

src/Serilog.Sinks.MSSqlServer/Configuration/Extensions/Microsoft.Extensions.Configuration/LoggerConfigurationMSSqlServerExtensions.cs

@@ -15,7 +15,6 @@
 using System;
 using Microsoft.Extensions.Configuration;
 using Serilog.Configuration;
-using Serilog.Core;
 using Serilog.Events;
 using Serilog.Formatting;
 using Serilog.Sinks.MSSqlServer;