Remove scr.rainbow option

[XVilka]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
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Detailed description

Removed scr.rainbow whose usefulness is highly doubtful also it messes colors from the colortheme:

With e scr.rainbow=true

Without scr.rainbow (the default state)

Test plan

CI is green

Partially addresses #4055