Del unused funcs

[oruebel]

Motivation

Move breaking changes from #2031 to this new branch/PR so they can be reviewed and merged separately.

Removed unused functions prepend_string and _not_parent in core.py, _not_parent in file.py, and NWBBaseTypeMapper.get_nwb_file in io/core.py

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.98%. Comparing base (bc12931) to head (81da051).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2036      +/-   ##
==========================================
+ Coverage   91.73%   91.98%   +0.25%     
==========================================
  Files          27       27              
  Lines        2722     2709      -13     
  Branches      710      707       -3     
==========================================
- Hits         2497     2492       -5     
+ Misses        149      141       -8     
  Partials       76       76              

Flag	Coverage Δ
integration	73.12% <100.00%> (+0.16%)	⬆️
unit	82.50% <100.00%> (+0.21%)	⬆️

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