Fix Regression from !300

In !300 (merged) all written field sizes are added to the SympyAssignment as unknown parameters. This solves the problem that all field sizes need to be passed as arguments when using NT stores with non-x86 architectures. However, it introduces two problems.

1. In all other cases these parameters are not used. Thus waLBerla fails in some cases when compiled with -Wall. Other than that it is not nice either to pass unused parameters.

2. For the GPU code generation problems arose with the usage of get_parameters in waLBerla: https://i10git.cs.fau.de/pycodegen/pystencils/-/blob/master/pystencils/astnodes.py#L244

Overall it seems that the easiest way to fix the problem is to only pass the additional size arguments when needed and in no other cases.