Get rid of unregistered `BrenierTwoFluid` in Documenter workflow

[benedict-96]

The functionality of BrenierTwoFluid should be copied to another package (perhaps in JuliaRCM) where we can maintain it¹. At the moment the unregistered package is installed every time the docs are generated.

Footnotes

1. This is probably not a top priority of @ToBlick who wrote the package. ↩