PromiseTask.isCancelled performs an unsynchronized read (#10066)

Motivation:

PromiseTask.isCancelled performs an unsynchronized read and so may trigger race-detectors. As this was just done for a small optimization which should not really make a lot of difference we should just remove it.

Modifications:

Remove unsynchronized read optimization

Result:

Fixes https://github.com/netty/netty/issues/10026.

common/src/main/java/io/netty/util/concurrent/PromiseTask.java

@@ -176,11 +176,6 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
         return clearTaskAfterCompletion(super.cancel(mayInterruptIfRunning), CANCELLED);
     }
 
-    @Override
-    public final boolean isCancelled() {
-        return task == CANCELLED || super.isCancelled();
-    }
-
     @Override
     protected StringBuilder toStringBuilder() {
         StringBuilder buf = super.toStringBuilder();