chore(celery): Remove transaction-based tracing

[sentrivana]

Removed transaction-based tracing from Celery. Also:

• Removed custom NoOpMgr and just use nullcontext now that we don't need to support 3.6 (nullcontext is 3.7+)
• Removed the skipped test_redis_backend_trace_propagation test. Originally I tried to adapt it to span streaming, but ran into the same issue with it hanging in CI.

Closes https://linear.app/getsentry/issue/PY-2685/remove-transaction-based-tracing-from-celery

[linear-code]

PY-2685

[github-actions]

Codecov Results 📊

✅ 80246 passed | ⏭️ 3573 skipped | Total: 83819 | Pass Rate: 95.74% | Execution Time: 258m 60s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -53887
Passed Tests	📉 -50338
Failed Tests	—
Skipped Tests	📉 -3549

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2263 uncovered lines.
❌ Project coverage is 90.22%. Comparing base (12ae986) to head (cc10e28).

Files with missing lines (1)

File	Patch %	Lines
sentry_sdk/integrations/celery/init.py	100.00%	⚠️ 2 partials

Coverage diff

@@            Coverage Diff             @@
##        master       #PR       +/-##
==========================================
- Coverage    90.25%    90.22%    -0.03%
==========================================
  Files          193       185        -8
  Lines        25725     23135     -2590
  Branches      9494      8474     -1020
==========================================
+ Hits         23217     20872     -2345
- Misses        2508      2263      -245
- Partials      1432      1349       -83

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