[FEA] update --mip-heuristics-only flag

[mlubin]

The --mip-heuristics-only flag should be updated to reflect the latest and greatest heuristic code paths that are used when branch and bound is enabled.

[chris-maes]

I think we should deprecate this flag. We added it in the beginning because branch and bound was experimental and we wanted to give users the option of turning it off.

We could add a setting that altered the solver so it focused on improving the primal bound. But I don't think it makes sense to completely turn off branch and bound anymore (which is what this flag does).