[diffcoeff] Fix occurences of diffCoeff(p,p,c) using FS::getMainComponent

FluidSystem::getMainComponent returns the main componet index of the phase.
This doesn't actually make sense for the tracer models which why this commit also
* removes getMainComponent for tracer fluid systems (Changelog amended)
* guards all occurences of getMainComponent in code that works for tracer too
with constexpr if