fix(celery): remove reference to github issue

[emmettbutler]

The issue referenced was resolved, but it actually does not apply to the bit of code it's pointing to.

Fixes #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 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.

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed.
• Documentation is included (in-code, generated user docs, public corp docs).
• Author is aware of the performance implications of this PR as reported in the benchmarks PR comment.

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer is aware of, and discussed the performance implications of this PR as reported in the benchmarks PR comment.

[codecov-commenter]

Codecov Report

Merging #5381 (b5702dc) into 1.x (d78d37c) will increase coverage by 0.00%.
The diff coverage is 89.47%.

@@           Coverage Diff           @@
##              1.x    #5381   +/-   ##
=======================================
  Coverage   76.53%   76.53%           
=======================================
  Files         887      887           
  Lines       70146    70158   +12     
=======================================
+ Hits        53685    53696   +11     
- Misses      16461    16462    +1     

Impacted Files	Coverage Δ
ddtrace/contrib/celery/signals.py	82.03% <ø> (ø)
ddtrace/appsec/ddwaf/__init__.py	74.33% <66.66%> (+0.22%)	⬆️
ddtrace/appsec/processor.py	92.72% <100.00%> (-0.04%)	⬇️
tests/appsec/test_processor.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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