Remove conda_build/index.py

[dholth]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

With standalone conda-index, conda-build's legacy (slow) index code is no longer needed.

Should we rewrite the conda index command in terms of standalone conda-index? Can we remove that too and use python3 -m conda_index's improved but different click-based CLI?

[dholth]

See #5152 ; index is deprecated.

[github-actions]

Hi there, thank you for your contribution!

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Thanks!

[beckermr]

Not stale.