Flix overlap

[styx0x6]

During the screening of the available built-in grammars (#4439), I noticed that Flix exists as a built-in grammar but is referenced in supported languages as a 3rd party lib.

The author of the built-in grammar seems to be originally a Flix maintainer. The 3rd reference is also from Flix directly and seems more up-to-date.

My proposal: simply remove flix.js as a built-in language in highlight.js.

[joshgoebel]

I'm not opposed to losing a grammar, but it would likely have to wait until v12 since it would be a breaking change.