remove unused reference

Author: zhiyuanliang-ms

src/Microsoft.Extensions.Configuration.AzureAppConfiguration/JsonKeyValueAdapter.cs

@@ -4,7 +4,6 @@
 using Azure.Data.AppConfiguration;
 using Microsoft.Extensions.Configuration.AzureAppConfiguration.Extensions;
 using Microsoft.Extensions.Configuration.AzureAppConfiguration.FeatureManagement;
-using Microsoft.Extensions.Options;
 using System;
 using System.Collections.Generic;
 using System.Net.Mime;