ref(tornado): Remove unused compat code

[sentrivana]

We're now enforcing Tornado > 6, so compat code for older versions is not needed anymore

[github-actions]

Codecov Results 📊

✅ 92339 passed | ⏭️ 5436 skipped | Total: 97775 | Pass Rate: 94.44% | Execution Time: 322m 58s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -5438
Passed Tests	📉 -4635
Failed Tests	—
Skipped Tests	📉 -803

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2325 uncovered lines.
✅ Project coverage is 90.18%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.91%    90.18%    +0.27%
==========================================
  Files          193       186        -7
  Lines        24828     23673     -1155
  Branches      8930      8538      -392
==========================================
+ Hits         22322     21348      -974
- Misses        2506      2325      -181
- Partials      1416      1360       -56

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Generated by Codecov Action

[Chenghao999]

Verified this removal is safe and consistent:

• The tornado extra in setup.py is pinned to tornado>=6, and the runtime floor check already enforces it via _MIN_VERSIONS["tornado"] = (6, 0) in sentry_sdk/integrations/__init__.py. So RequestHandler._execute is guaranteed to be a native coroutine whenever this integration can run: the iscoroutinefunction branch was always True and the generator path was dead code.
• tests/integrations/tornado/test_tornado.py contains no references to the removed path, which is consistent with it being unreachable, and patch coverage is 100%.
• The import removals are complete — nothing else in the file references iscoroutinefunction or tornado.gen.

Optional nit: the "Starting Tornado 6 ..." comment disappeared together with the branch. A one-liner on the now-unconditional async def wrapper explaining that Tornado >= 6 (enforced both in the extras and at runtime) makes the compat path unnecessary would help future readers.

LGTM — nice small cleanup.