rf(schema): Drop context.dataset.files

[effigies]

While working on the validator, I realized context.dataset.files was never being populated, and didn't need to be. Removing the couple of uses from the rules and then removing context.dataset.files altogether.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.06%. Comparing base (d2557e3) to head (1dc3d00).
Report is 283 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1883   +/-   ##
=======================================
  Coverage   88.06%   88.06%           
=======================================
  Files          16       16           
  Lines        1391     1391           
=======================================
  Hits         1225     1225           
  Misses        166      166           

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