Purge deno validator

[effigies]

Removes the deno validator, which will be in its own repository.

The current plan is to rename this repository to bids-standard/legacy-validator and move a fork containing only the schema validator to bids-standard/bids-validator.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.70%. Comparing base (5192ed0) to head (2b75851).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2173   +/-   ##
=======================================
  Coverage   85.70%   85.70%           
=======================================
  Files          91       91           
  Lines        3784     3784           
  Branches     1221     1221           
=======================================
  Hits         3243     3243           
  Misses        455      455           
  Partials       86       86           

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