Remove references to defunct git_ignore config

[sparrowt]

https://pycqa.github.io/isort/docs/configuration/options.html#git-ignore is nonsense and doesn't work - it raises UnsupportedSettings because e168069 removed it without removing the documentation that refers to it.

The remaining config option that still works is skip_gitignore https://pycqa.github.io/isort/docs/configuration/options.html#skip-gitignore.

Remove the defunct one to avoid confusion.

Fixes #1920

[Foryah]

It's true that git_ignore was removed but I think it's more accurate to say it was replaced with git_ls_files as we can see in the commit. This PR solves half the problem, to solve #1920 we still need to document how git_ls_files works 😁

[sparrowt]

I think the real question is whether git_ls_files should really be in _Config at all - AFAICS it's more of an implementation detail, caching the result of calling git ls-files (which is likely to be pretty big for most repos) and something which I can't imagine anyone wanting to pass in as a config.

So I think this PR probably does all of the documentation changes that we want related to this - perhaps a separate ticket is warranted for whether git_ls_files should be moved out of the config class seeing as that doesn't quite seem appropriate, but I'm new to this codebase so may have missed something & didn't want to go trying to move functional code around, to keep this "avoid confusion" PR simple.

[Foryah]

Makes a lot of sense 😁 I never read the code other than the commit you passed and there I just saw that it was a replacement so I assumed is similar funtionality, different flag. But yeah, if the functionality is different, then all good 🥳

[DanielNoord]

Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.15%. Comparing base (ab9cd36) to head (a53fecb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2531   +/-   ##
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  Coverage   99.15%   99.15%           
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  Files          41       41           
  Lines        3090     3090           
  Branches      668      668           
=======================================
  Hits         3064     3064           
  Misses         14       14           
  Partials       12       12           

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