[cleanup] Remove unused PengRobinsonMixture::computeMolarVolumes

It also wouldn't translate because PengRobinson::computeMolarVolumes
does not exist.
5 jobs for !2813 with cleanup/computemolarvolumes-peng-robinson
latest detached
Status Name Job ID Coverage
  Trigger
passed full-dune-2.7-clang

passed full-dune-2.7-gcc

passed full-dune-master-gcc

passed minimal-dune-2.7-gcc

 
  Downstream Modules
passed trigger lecture