remove subscription cache field

Author: rose-a

src/GraphQL.Client/GraphQLHttpClient.cs

@@ -1,11 +1,9 @@
 using System;
-using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using System.Net.Http;
 using System.Net.Http.Headers;
-using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 using GraphQL.Client.Abstractions;
@@ -20,7 +18,6 @@ public class GraphQLHttpClient : IGraphQLClient
         private GraphQLHttpWebSocket GraphQlHttpWebSocket => _lazyHttpWebSocket.Value;
 
         private readonly CancellationTokenSource _cancellationTokenSource = new CancellationTokenSource();
-        private readonly ConcurrentDictionary<Tuple<GraphQLRequest, Type>, object> _subscriptionStreams = new ConcurrentDictionary<Tuple<GraphQLRequest, Type>, object>();
 
         private readonly bool _disposeHttpClient = false;