Remove dead Opset function_defs registration

[gramalingam]

The function_defs dict on Opset and add_function_def() method are written to but never read from anywhere in the codebase. Removing this dead code eliminates a memory leak where the singleton Opset holds references to all compiled functions forever.

This will also help simplify further cleanups (relating to better handling of conversion of script functions to IR ... which currently goes from IR to proto to IR unnecessarily).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.48%. Comparing base (1911741) to head (250686d).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2901      +/-   ##
==========================================
- Coverage   72.51%   72.48%   -0.04%     
==========================================
  Files         250      258       +8     
  Lines       30955    31471     +516     
  Branches     2960     2973      +13     
==========================================
+ Hits        22448    22811     +363     
- Misses       7504     7650     +146     
- Partials     1003     1010       +7     

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