refactor: Remove the unused retry classification helper

[vdusek]

_utils/errors.py::is_retryable_error is a stale copy of http_clients/_impit.py::_is_retryable_error. No production code calls it — only its own tests did — and it sits in a private module, so nothing depends on it. Once #1019 narrows the real policy, keeping a second copy that still claims every impit.HTTPError is transient is actively misleading.

Removes the function, its two test cases, and the imports they alone needed.

Split out of #1006.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (334e131) to head (b0939e3).

Additional details and impacted files

@@ Coverage Diff @@## master #1021 +/- ##
==========================================
- Coverage 94.87% 94.87% -0.01% 
==========================================
Files 58 58 Lines 5367 5364 -3 ==========================================
- Hits 5092 5089 -3 
Misses 275 275 

Flag	Coverage Δ
integration	92.20% <100.00%> (-0.01%)	⬇️
unit	85.14% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.