[readme] remove `avn` from readme

[andrew-chang-dewitt]

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avn is archived (see repo) and should probably no longer be recommended by nvm.

[andrew-chang-dewitt]

this is a pretty big chunk of code to include. probably should be wrapped in a <details>, but either way i think it needs to be polished a bit, and made POSIX-compliant (ie, no bash-specific syntax)

Good point. I think it might actually be even better to limit this PR to avn no longer being maintained & remove the direnv recommendation. I might put it in a second PR and open it later after some testing.

[ljharb]

That sounds like a great plan!

[andrew-chang-dewitt]

Should have everything not about avn removed from this now.