Replace unnecessary CHOLMOD_LIBS flag with LINSOLV
Replacing unnecessary CHOLMOD_LIBS flag with cmake-step-defined and already CHOLMOD dependent LINSOLV flag. It cleans the code and fixes launchpad issue, where solvers >0 where not compiled since CHOLMOD_LIBS flag was not explctly defined.
This change might have consequential blindspots in various flow engines (TwoPhaseFlow?) despite successful compilation, therefore it remains WIP until approved by @bchareyre. Automatic and manual testing shows full functionality remains.