Issue #3246 - remove debugging code from cherry-picked commit

Signed-off-by: Lachlan Roberts <lachlan@webtide.com>

jetty-io/src/main/java/org/eclipse/jetty/io/WriteFlusher.java

@@ -55,8 +55,6 @@ abstract public class WriteFlusher
     private final EndPoint _endPoint;
     private final AtomicReference<State> _state = new AtomicReference<>();
 
-    Throwable last;
-
     static
     {
         // fill the state machine
@@ -272,13 +270,7 @@ abstract public class WriteFlusher
             LOG.debug("write: {} {}", this, BufferUtil.toDetailString(buffers));
 
         if (!updateState(__IDLE, __WRITING))
-        {
-            if (last!=null)
-                last.printStackTrace();
             throw new WritePendingException();
-        }
-
-        last = new Throwable();
 
         try
         {