CLN: dataset: Drop deprecated get_subdatasets()

[kyleam]

This has been marked as deprecated since v0.9.0.

[codecov]

Codecov Report

Merging #3336 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3336      +/-   ##
==========================================
- Coverage   91.13%   91.12%   -0.02%     
==========================================
  Files         263      263              
  Lines       34126    34119       -7     
==========================================
- Hits        31100    31090      -10     
- Misses       3026     3029       +3

Impacted Files	Coverage Δ
datalad/distribution/dataset.py	96.78% <ø> (+2.01%)	⬆️
datalad/downloaders/tests/test_http.py	88.86% <0%> (-2.23%)	⬇️

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