fix(api): Dataset deletion would not remove the MongoDB record correctly

[nellh]

This collection uses id for datasetId and this will silently not delete anything when a dataset is removed. Aggregates will then accidentally include removed datasets.

Fixes #3590 and will require a database update to remove the existing failures once this has been deployed.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 42.93%. Comparing base (ae223fd) to head (6cfa395).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/openneuro-server/src/datalad/dataset.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3594      +/-   ##
==========================================
+ Coverage   41.15%   42.93%   +1.78%     
==========================================
  Files         582      633      +51     
  Lines       31182    33142    +1960     
  Branches     1479     1479              
==========================================
+ Hits        12833    14231    +1398     
- Misses      18210    18772     +562     
  Partials      139      139              

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