Remove misleading FSLeyes installation instructions

[joshuacwnewton]

Checklist

GitHub

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
• I've applied the relevant labels to this PR
• I've applied a release milestone (major, minor, patch) in line with Semantic Versioning guidelines
• I've assigned a reviewer

PR contents

• I've consulted SCT's internal developer documentation to ensure my contribution is in line with any relevant design decisions
• I've added relevant tests for my contribution
• I've updated the relevant documentation for my changes, including argparse descriptions, docstrings, and ReadTheDocs tutorial pages

Description

While going over our installation pages, I noticed the installation/fsleyes.rst page that I had never seen before. But:

• FSLeyes should not be installed in the SCT venv (see this issue), so these instructions could mislead users into breaking their SCT installation.
• I believe that the "FSLeyes Integration" page is sufficient, and that we do not need a second page.

Linked issues

Fixes #3223.

[jcohenadad]

@joshuacwnewton i've added label/milestone-- feel free to disregard

[joshuacwnewton]

@joshuacwnewton i've added label/milestone-- feel free to disregard

Perfect! I was rushing a bit, so thank you. 🙂