rebase and address feedback

Author: Wraith2

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/AAsyncCallContext.cs

@@ -42,7 +42,6 @@ protected override void DisposeCore()
         {
             TDisposable copyDisposable = _disposable;
             _disposable = default;
-            _isDisposed = true;
             copyDisposable?.Dispose();
         }
     }

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlDataReader.cs

@@ -4734,7 +4734,7 @@ public override Task<bool> ReadAsync(CancellationToken cancellationToken)
                 }
 
                 // Register first to catch any already expired tokens to be able to trigger cancellation event.
-                IDisposable registration = null;
+                CancellationTokenRegistration registration = default;
                 if (cancellationToken.CanBeCanceled)
                 {
                     registration = cancellationToken.Register(SqlCommand.s_cancelIgnoreFailure, _command);
@@ -4838,12 +4838,6 @@ public override Task<bool> ReadAsync(CancellationToken cancellationToken)
                     return source.Task;
                 }
 
-                CancellationTokenRegistration registration = default;
-                if (cancellationToken.CanBeCanceled)
-                {
-                    registration = cancellationToken.Register(SqlCommand.s_cancelIgnoreFailure, _command);
-                }
-
                 ReadAsyncCallContext context = null;
                 if (_connection?.InnerConnection is SqlInternalConnection sqlInternalConnection)
                 {