index: remove unused APIs

[skshetry]

These are being used by some users, but removing them for easier refactoring for now.
We are not using it ourselves.

[codecov]

Codecov Report

Base: 93.53% // Head: 93.52% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (793bb6b) compared to base (198daec).
Patch coverage: 88.88% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8712      +/-   ##
==========================================
- Coverage   93.53%   93.52%   -0.01%     
==========================================
  Files         457      457              
  Lines       36265    36128     -137     
  Branches     5237     5222      -15     
==========================================
- Hits        33920    33790     -130     
+ Misses       1844     1837       -7     
  Partials      501      501              

Impacted Files	Coverage Δ
dvc/repo/index.py	93.03% <75.00%> (+1.08%)	⬆️
dvc/repo/stage.py	90.84% <100.00%> (ø)
tests/func/test_repo_index.py	95.13% <100.00%> (-1.70%)	⬇️

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