Remove legacy tests._helpers.py

[jakob-keller]

Description of Change

Remove legacy tests._helpers.py

Assumptions

Content of removed module no longer relevant for supported Python versions.

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.21%. Comparing base (aa44442) to head (453a494).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1145      +/-   ##
==========================================
+ Coverage   86.19%   86.21%   +0.02%     
==========================================
  Files          63       62       -1     
  Lines        5955     5950       -5     
==========================================
- Hits         5133     5130       -3     
+ Misses        822      820       -2     

Flag	Coverage Δ
unittests	86.21% <100.00%> (+0.02%)	⬆️

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