Temporarily revert "add age__reference and adjust tests (#1540)"

[rly]

This reverts commit 5d8efdb.

We accidentally merged #1540 into the dev branch instead of to a new branch for NWB 2.6 support.

I will create this new branch and move this commit to that branch.

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 #1610 (a0249b9) into dev (f4bd3bc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1610   +/-   ##
=======================================
  Coverage   91.25%   91.25%           
=======================================
  Files          25       25           
  Lines        2515     2515           
  Branches      477      477           
=======================================
  Hits         2295     2295           
  Misses        139      139           
  Partials       81       81           

Flag	Coverage Δ
integration	70.21% <0.00%> (ø)
unit	84.49% <0.00%> (ø)

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

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more