Update to remove smimesign from Linux instructions

[N6UDP]

Why:

#31465 was rejected because it added docs for gpgsm (X509 signing that works on Linux) - this at least clears up the fact smimesign does not work on Linux.

Closes:
github/smimesign#45
Related to:
github/smimesign#30

What's being changed (if available, include any code snippets, screenshots, or gifs):

Remove smimesign from Linux docs

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• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[N6UDP]

@jc-clark PR as requested

[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

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Source	Preview	Production	What Changed
authentication/managing-commit-signature-verification/telling-git-about-your-signing-key.md	fpt ghec ghes@ 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.12 3.11 3.10 3.9

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

@N6UDP Thanks so much for opening a PR with this fix! I'll get this merged 🍏

[github-actions]

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