Remove deprecated "--index/--no-index" option from compile command

[jdufresne]

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[codecov]

Codecov Report

Merging #1234 (4b5ff05) into master (f54ad96) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
- Coverage   99.62%   99.62%   -0.01%     
==========================================
  Files          33       33              
  Lines        2953     2926      -27     
  Branches      322      319       -3     
==========================================
- Hits         2942     2915      -27     
  Misses          5        5              
  Partials        6        6              

Impacted Files	Coverage Δ
tests/test_utils.py	99.21% <ø> (ø)
piptools/scripts/compile.py	100.00% <100.00%> (ø)
tests/test_cli_compile.py	100.00% <100.00%> (ø)

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[atugushev]

@jazzband/pip-tools

Hey, everyone! I'm on board to merge this PR and remove --index/--no-index in 6.0.0, it's been 9 months since the option was deprecated in 5.2.0. Let me know If anyone is opposed to this.

[atugushev]

👍🏼