Remove references to region references/builders for HDMF 4.0

[rly]

Motivation

HDMF 4.0 and hdmf-schema-language 3.0 are removing support for region references. There is an unused IO objectmapper class that refers to RegionBuilder, and a docs page that refers to RegionBuilder. This PR removes those.

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 92.04%. Comparing base (969c65f) to head (682cf71).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1991      +/-   ##
==========================================
+ Coverage   91.92%   92.04%   +0.12%     
==========================================
  Files          27       27              
  Lines        2713     2704       -9     
  Branches      708      705       -3     
==========================================
- Hits         2494     2489       -5     
+ Misses        145      141       -4     
  Partials       74       74              

Flag	Coverage Δ
integration	72.92% <100.00%> (+0.05%)	⬆️
unit	82.98% <100.00%> (+0.09%)	⬆️

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