Remove the @builtin group selector

[j178]

Keeps @ungrouped as the only special group selector.

Follow-up to #2617.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.97%. Comparing base (67d2254) to head (669c8ce).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2628   +/-   ##
=======================================
  Coverage   93.97%   93.97%           
=======================================
  Files         140      140           
  Lines       29719    29715    -4     
=======================================
- Hits        27928    27925    -3     
+ Misses       1791     1790    -1     

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