Remove nightly-only options in rustfmt config

[romac]

These options cannot be used on stable Rust, which we use for developing the library and on CI for checking the formatting. As such they should be removed from the .rustfmt.toml config, as they can otherwise be picked up by some IDEs which will automatically use the nightly rustfmt to honor the unstable options.

[codecov-commenter]

Codecov Report

Merging #1303 (6b2bf4f) into main (9f0ee2b) will not change coverage.
The diff coverage is n/a.

❗ Current head 6b2bf4f differs from pull request most recent head 6718edc. Consider uploading reports for the commit 6718edc to get more accurate results

@@          Coverage Diff          @@
##            main   #1303   +/-   ##
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  Coverage   64.2%   64.2%           
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  Files        271     271           
  Lines      24472   24472           
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  Hits       15732   15732           
  Misses      8740    8740           

see 4 files with indirect coverage changes

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[thanethomson]

Any idea when these formatting options will make their way into stable? This file's been configured like this for ages now, and these formatting conventions have served us pretty well.

But I'm also all for not depending on nightly 🙂

[romac]

We're actually formatting and checking the code with the stable toolchain so those had no effect up to now afaict. At least it on my machine, running cargo fmt or formatting with rust-analyzer on stable does not pick them up.

I only noticed this because my new Neovim distrib (AstroNvim) seems to use nightly rustfmt by default, hence this PR to ensure it does not reformat all the code I touch using the unstable settings.

[romac]

No idea as to when these options will become stable but I wouldn't hold my breath given that they've unstable for years now.