[Chess] Simplify legal_underpromotions

[sotetsuk]

#1234

fn	jaxpr lines	time
_legal_action_mask	1631	485.1ms
_flip	91	25.2ms
_is_checked	203	53.4ms
_is_pseudo_legal	134	35.0ms
_hash_castling_en_passant	37	17.4ms
_apply_move	258	69.3ms
_update_history	57	28.6ms

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.59%. Comparing base (27db7ba) to head (6d52b2e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1236      +/-   ##
==========================================
- Coverage   80.61%   80.59%   -0.02%     
==========================================
  Files          62       62              
  Lines        5957     5953       -4     
==========================================
- Hits         4802     4798       -4     
  Misses       1155     1155              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.