optimization disposeAsync

Author: izanhzh

src/MiniExcel/IMiniExcelDataReader.cs

@@ -28,9 +28,5 @@ public interface IMiniExcelDataReader : IDataReader, IAsyncDisposable
         Task<bool> ReadAsync();
 
         Task<bool> ReadAsync(CancellationToken cancellationToken);
-
-#if !NET8_0_OR_GREATER
-        Task DisposeAsync();
-#endif
     }
 }

src/MiniExcel/MiniExcelDataReaderBase.cs

@@ -393,16 +393,6 @@ public virtual ValueTask DisposeAsync()
             Dispose();
             return default;
         }
-#else
-        /// <summary>
-        /// <inheritdoc/>
-        /// </summary>
-        /// <returns></returns>
-        public virtual Task DisposeAsync()
-        {
-            Dispose();
-            return MiniExcelTask.CompletedTask;
-        }
 #endif
 
         /// <summary>