Remove unused using directive

Deleted an unnecessary directive for System.Xml.Schema in NatsJSContext.cs. This cleanup aids in maintaining clean and efficient code.

src/NATS.Client.JetStream/NatsJSContext.cs

@@ -1,6 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
-using System.Xml.Schema;
 using Microsoft.Extensions.Logging;
 using NATS.Client.Core;
 using NATS.Client.JetStream.Internal;