PCFieldSplit: remove unused is_cols data and remove redundant argument from PCFieldSplitSetFields

PCFieldSplit has internal is_cols and fields_cols data, but it's not clear that these provide any functionality, and removing them as in this MR seems to have no effect on the library (and should improve performance by removing seemingly-redundant operations).

I'm not sure what this data is supposed to represent - it seems possible that this is a confusion between the splitting of the problem unknowns/DOF and the partitioning of a Mat (e.g. is_cols appears in the MatNest implementation).

This was added in 5d4c12cd, presumably to provide functionality for a project at the time, but I haven't been able to find additional context yet. Without that I'm of course concerned that I've overlooking some subtlety

These changes involve changing the API to remove the last argument of PCFieldSplitSetFields().