Remove PETSc time solver #3126
Description
From a quick read of the source, the PETSc time solver just switches between Adams-Moulton or BDF via SUNDIALS (specifically a "very old version" of CVODE according to the docs), or Runge-Kutta. Also the colouring almost certainly has never worked correctly.
Given that we have our own interface to both recent CVODE and RK, it makes sense to completely remove this solver. Any objections?