revert: "feat: add flag to skip deleting obsolete referrers index"

[qweeah]

Related to #1046

[codecov-commenter]

Codecov Report

Merging #1060 (44c61b9) into main (ceb56f8) will decrease coverage by 0.30%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1060      +/-   ##
==========================================
- Coverage   81.26%   80.96%   -0.30%     
==========================================
  Files          57       56       -1     
  Lines        2914     2890      -24     
==========================================
- Hits         2368     2340      -28     
- Misses        373      375       +2     
- Partials      173      175       +2     

Files Changed	Coverage Δ
cmd/oras/internal/errors/errors.go	100.00% <ø> (ø)
cmd/oras/root/attach.go	82.50% <100.00%> (-2.99%)	⬇️
cmd/oras/root/cp.go	86.36% <100.00%> (-0.50%)	⬇️
cmd/oras/root/manifest/push.go	74.41% <100.00%> (-1.34%)	⬇️

[shizhMSFT]

LGTM