Remove legacy flags and helpers

[aaronlew02]

Depends on #5109

Summary

This change removes flags and helpers obsoleted by the removal of the legacy (non-new-bundle-format) signing and verification codepaths.

[codecov]

Codecov Report

❌ Patch coverage is 52.47934% with 115 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.21%. Comparing base (2ef6022) to head (4034ce9).
⚠️ Report is 879 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/cosign/cli/attach/attach.go	0.00%	14 Missing ⚠️
cmd/cosign/cli/attest.go	23.07%	10 Missing ⚠️
cmd/cosign/cli/attest_blob.go	47.36%	10 Missing ⚠️
cmd/cosign/cli/verify/verify.go	37.50%	10 Missing ⚠️
cmd/cosign/cli/verify.go	0.00%	9 Missing ⚠️
cmd/cosign/cli/attest/attest.go	0.00%	8 Missing ⚠️
cmd/cosign/cli/sign/sign_blob.go	41.66%	7 Missing ⚠️
cmd/cosign/cli/verify/verify_blob_attestation.go	80.00%	7 Missing ⚠️
pkg/cosign/verify.go	72.72%	6 Missing ⚠️
cmd/cosign/cli/sign/sign.go	0.00%	5 Missing ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5145      +/-   ##
==========================================
+ Coverage   40.10%   48.21%   +8.11%     
==========================================
  Files         155      205      +50     
  Lines       10044    10461     +417     
==========================================
+ Hits         4028     5044    +1016     
+ Misses       5530     5417     -113     
+ Partials      486        0     -486     

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