fix: remove NJ taxable_social_security double exclusion

[MaxGhenis]

Summary

• Remove taxable_social_security from NJ subtractions parameter list (now empty)
• Social Security was already excluded from NJ gross income (not in gross_income_sources.yaml) per NJ Statute 54A:6-15
• The prior subtraction created a double exclusion, incorrectly reducing taxable income

Test plan

• Added test case: Single, age 75, $37,274 wages, $27,262 gross SS
• Verified NJ taxable income = $35,274 (wages - $2,000 exemptions, NOT reduced by SS)
• Verified NJ tax = $552 (per TaxAct)
• Test passes locally

Fixes #6979

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Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.35%. Comparing base (66ccff2) to head (99db630).
⚠️ Report is 430 commits behind head on main.

Files with missing lines	Patch %	Lines
...ncome/adjusted_gross_income/nj_agi_subtractions.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7281      +/-   ##
==========================================
+ Coverage   72.72%   82.35%   +9.63%     
==========================================
  Files        3757        1    -3756     
  Lines       54115       17   -54098     
  Branches      267        1     -266     
==========================================
- Hits        39353       14   -39339     
+ Misses      14742        2   -14740     
+ Partials       20        1      -19     

Flag	Coverage Δ
unittests	82.35% <50.00%> (+9.63%)	⬆️

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