Remove unnecessary using GitHub.Copilot.SDK from Rpc.cs generator

Rpc.cs is in namespace GitHub.Copilot.SDK.Rpc, a child of
GitHub.Copilot.SDK, so types from the parent namespace resolve
automatically without an explicit using directive.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

dotnet/src/Generated/Rpc.cs

@@ -1197,7 +1197,6 @@ using System.ComponentModel.DataAnnotations;
 using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Serialization;
-using GitHub.Copilot.SDK;
 using StreamJsonRpc;
 
 namespace GitHub.Copilot.SDK.Rpc;