Remove OS buffer remark from FileStream.FlushAsync (#2532)

Per dotnet/coreclr#24902.

Author: stephentoub

xml/System.IO/FileStream.xml

@@ -2474,8 +2474,6 @@ Closes the current stream and releases any resources (such as sockets and file h
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- When you call the <xref:System.IO.FileStream.FlushAsync%2A> method, the operating system I/O buffer is also flushed.  
-  
  If the operation is canceled before it completes, the returned task contains the <xref:System.Threading.Tasks.TaskStatus.Canceled> value for the <xref:System.Threading.Tasks.Task.Status%2A> property. If the handle to the file is disposed, the returned task contains the <xref:System.ObjectDisposedException> exception in the <xref:System.Threading.Tasks.Task.Exception%2A> property.  
   
  ]]></format>
@@ -3923,4 +3921,4 @@ If the absolute path is not known, this property returns a string similar to "[U
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>