[Cleanup] Enforce dispatch policies used in generated kernels.

[wrtobin]

Change the policy at e.g.

https://github.com/GEOSX/GEOSX/blob/7f543eaf2ae3c9ce9bead7678ade167fa7bfbd4e/src/coreComponents/physicsSolvers/solidMechanics/SolidMechanicsLagrangianFEM.cpp#L268

And compile / link the code on a platform with gpu dispatch, and the code will fail to link with the easiest error message ever to parse.

This is due to our generated kernels having their policies explicitly specified in e.g.

https://github.com/GEOSX/GEOSX/blob/7f543eaf2ae3c9ce9bead7678ade167fa7bfbd4e/src/coreComponents/physicsSolvers/solidMechanics/kernels/SolidMechanicsKernels.cmake#L7-L10

This probably won't be an immediate issue until / unless we work to change these depending on the platform. Since there is currently no enforcement that the generated kernels have policies matching the usage elsewhere in the codebase.