Fix `z-index` for inline findings gutter icon

What does this MR do and why?

Remove gl-z-index-200 from inline findings icon

This keeps the icon from passing over the sticky file title when scrolling
because diffs.scss sets the z-index of the .file-title to 120

Changelog: fixed
EE: true

MR acceptance checklist

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Screenshots or screen recordings

Screen_Recording_2024-02-14_at_16.31.50

Note: This also happens with the contents of the dropdown, but this MR only fixes the icon. I've opened an issue for the dropdown panel 👉🏻 #441955

How to set up and validate locally

reproduce the bug 🐛

0. git checkout master
1. clone/make a project that generates code quality results - I recommend this one
2. make changes that result in new code quality violations in a new merge request, like this one
3. ensure pipelines ran for the base and head commits of the merge request's branch
4. scroll down through the diff and notice that the severity icons go in front of the file title panel when it sticks to the top

test the fix

5. git checkout fix-z-index-for-inline-findings-gutter-icon-dropdown and refresh
6. scroll down again, the icons should now go behind the file title panel when it sticks to the top 🎉