Remove redundant mapper functions

[rly]

Motivation

See NeurodataWithoutBorders/helpdesk#37 (reply in thread)

These name mapper functions are handled by ObjectMapper automatically and appear to be legacy code that adds confusion.

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 flake8 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

Merging #1600 (98c5349) into dev (a4f24bc) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1600      +/-   ##
==========================================
- Coverage   91.31%   91.28%   -0.03%     
==========================================
  Files          25       25              
  Lines        2510     2502       -8     
  Branches      472      472              
==========================================
- Hits         2292     2284       -8     
  Misses        138      138              
  Partials       80       80              

Flag	Coverage Δ
integration	70.38% <100.00%> (-0.10%)	⬇️
unit	84.45% <100.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
src/pynwb/io/base.py	83.95% <ø> (-1.11%)	⬇️
src/pynwb/io/core.py	78.84% <100.00%> (-0.79%)	⬇️

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