Timo Koch authored Mar 25, 2020 and Ned Coltman committed Mar 26, 2020

The idea of the tracer model is really tied to Fickian diffusion and components
with small concentration diffusing in a bulk phase. Hence also it's name.
The Maxwell-Stefan test actual models 3 gaseous components where none of
the components is really the main phase but most importantly there is no
bulk phase with a main component that is not considered in the equations.
Thus the 1pnc model is the right choice for such a test.
This changes the test to derive from the 1pnc model. As for the given
setting with only Neumann boundaries the 1pnc wouldn't convergence for
a compressible fluid, the Ideal gas law is used to model a
compressible gas.

The same test actually exists for the 1pnc model already in 1pnc/implicit/1p3c.
Thus this commit simply removes the tracer test.