Remove groupbydoc.

[bdice]

Description

Remove deprecated groupbydoc method and consolidate tests.

Motivation and Context

signac 2.0 API cleanup.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #601 (02a5a5d) into next (6290256) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             next     #601      +/-   ##
==========================================
- Coverage   85.78%   85.77%   -0.01%     
==========================================
  Files          52       52              
  Lines        5099     5075      -24     
  Branches      921      914       -7     
==========================================
- Hits         4374     4353      -21     
+ Misses        524      522       -2     
+ Partials      201      200       -1     

Impacted Files	Coverage Δ
signac/contrib/project.py	88.29% <ø> (+0.02%)	⬆️

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