Remove old fix for LuaLaTeX -- this is baked into Server Pro 3.2 onwards

[das7pad]

Description

I think it's time to remove the config hack from 2020. Any releases from 3.2 onwards will have the env var baked in: https://github.com/overleaf/overleaf/pull/739.

I think it's OK to temporarily break LuaLaTeX for compiles when customers upgrade the toolkit in a <3.2 setup (which is not supported anymore, for anything other than performing an upgrade).

Related issues / Pull Requests

https://github.com/overleaf/overleaf/pull/739

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