fix(celery): remove reference to github issue (DataDog#5381)

The issue referenced was
[resolved](celery/celery#6374), but it actually
does not apply to the bit of code it's pointing to.

Fixes DataDog#4649

I tried removing the code referenced by the comment, and it causes the
test
`tests/contrib/celery/test_integration.py::CeleryDistributedTracingIntegrationTask::test_distributed_tracing_propagation_async`
to fail. As far as I can tell from about an hour of investigation, the
celery [fix](celery/celery#4356) only affects
"hybrid messages" (those that blend protocol 1 and 2 syntax as part of a
rolling Celery upgrade), which our test suite doesn't use.

ddtrace/contrib/celery/signals.py

@@ -134,9 +134,7 @@ def trace_before_publish(*args, **kwargs):
         trace_headers = {}
         propagator.inject(span.context, trace_headers)
 
-        # This weirdness is due to yet another Celery bug concerning
-        # how headers get propagated in async flows
-        # https://github.com/celery/celery/issues/4875
+        # put distributed trace headers where celery will propagate them
         task_headers = kwargs.get("headers") or {}
         task_headers.setdefault("headers", {})
         task_headers["headers"].update(trace_headers)