Remove uses_forcing_file and forcing_file options

[mattw-nws]

These features were added to support explicitly providing a forcing file path to a BMI model, but no mechanism was ever found/created to accomplish that, and currently the forcing_file value gets stored in the BMI_Adapter<T> and never used. Further, formulations with no forcing input is going to be provided by #569. These should be removed for clarity and maintainability. Documentation should also be updated to remove references. Notably, existing formulations that have this key in their JSON (at least as false) will not break, the key will just be ignored. However, example formulations in the repo should probably have these keys removed from the JSON. There are some tests on this feature (and static friend functions to support them) that will also need to be removed.