Remove `new_fonts` feature flag
What does this MR do and why?
The feature flag was enabled on prod by default for a month now. We have fixed all major issues and are ready for the FF removal.
How to set up and validate locally
Here is a link to the Stack Overflow answer which describes how to see what font is used for the element in different browsers. Checking one browser would be enough though))
Sans serif (most of the text) should use GitLab Sans
where monospace text shouls use JetBrains Mono
Related to #379147 (closed)