fix(schema): Remove task entity selector for associated events

[effigies]

As of #1440, events files are no longer assumed to be associated with task data files (although currently all events files do have a required task entity). This brings meta.associations.events into line, so that events files can be found.

This is somewhat abstract and will come at a slight cost to the validator, but other tools using meta.associations to find associated files will benefit from removing the restriction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.71%. Comparing base (2ca8e81) to head (6baefcb).
⚠️ Report is 18 commits behind head on maint/1.10.1.

Additional details and impacted files

@@              Coverage Diff              @@
##           maint/1.10.1    #2202   +/-   ##
=============================================
  Coverage         82.71%   82.71%           
=============================================
  Files                20       20           
  Lines              1608     1608           
=============================================
  Hits               1330     1330           
  Misses              278      278           

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[VisLab]

I think that task should be removed as a required entity for events from a number of the modality specifications, but this is a task for the future.