DM-31769 Remove Placeholder from DatasetType

[natelust]

Previously a placeholder was created to aid in graph creation
involving component dataset types. Graph creation code was modified
such that this no longer needs to be an attribute of DatasetType.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #586 (c5f4ef8) into master (f03b063) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
- Coverage   83.55%   83.53%   -0.02%     
==========================================
  Files         241      241              
  Lines       30253    30228      -25     
  Branches     4512     4509       -3     
==========================================
- Hits        25277    25252      -25     
  Misses       3784     3784              
  Partials     1192     1192              

Impacted Files	Coverage Δ
python/lsst/daf/butler/core/datasets/type.py	81.25% <ø> (-0.93%)	⬇️
tests/test_datasets.py	99.19% <ø> (-0.05%)	⬇️
tests/test_templates.py	98.75% <100.00%> (+<0.01%)	⬆️

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