Docs: remove Vim setup from GitLab install instructions

What does this MR do?

This MR removes the optional Vim setup from "GitLab install by source" instructions.

I have come up with four cases, each of which results in the Vim setup instructions being unnecessary:

1. user is experienced with Vim and already has it set up (no action needed, instructions have no value)
2. user is experienced with Vim and doesn't have it set up (user knows how to set it up/can find instructions online, instructions have little value)
3. user is not experienced with Vim and sets it up (user becomes frustrated using Vim because of the learning curve)
4. user is not experienced with Vim and won't set it up (no action needed, instructions have no value)

Related issues

Closes #288852 (closed).

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• Follow the Documentation Guidelines and Style Guide.
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1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
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