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%)	⬇️

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