remove_dotdot_prefixes: remove bad assert, fixes #9406

[ThomasWaldmann]

Description

remove bad assertion.

guess the assert was meant to make sure that we do not have backslashes as path separators, but did not consider that on linux a backslash can be part of a filename (without being a path separator).

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.47%. Comparing base (4344eb3) to head (23f7388).
⚠️ Report is 8 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9407      +/-   ##
==========================================
- Coverage   76.47%   76.47%   -0.01%     
==========================================
  Files          85       85              
  Lines       14804    14803       -1     
  Branches     2213     2213              
==========================================
- Hits        11322    11321       -1     
  Misses       2803     2803              
  Partials      679      679              

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