Remove redundant registration of operator::add and fix sub.Scalar

[justinchuby]

I forgot to remove the previous registration. Also fix aten.sub.Scalar.

Although I don't think we see the Scalar overload for these ops because we run the FX type promotion pass.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.38%. Comparing base (dd14682) to head (4fdd167).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/core.py	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2631      +/-   ##
==========================================
- Coverage   70.39%   70.38%   -0.01%     
==========================================
  Files         222      222              
  Lines       26281    26289       +8     
  Branches     2629     2629              
==========================================
+ Hits        18500    18504       +4     
- Misses       6861     6865       +4     
  Partials      920      920              

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