Remove DESIGN_MATRIX_GROUP label from design matrix config

[xjules]

Upon retrospect the keyword that is created in design matrix config:

parameter_configuration[DESIGN_MATRIX_GROUP] = GenKwConfig(...)

is not really used anyway and as such parameter configuration could directly refer to GenKwConfig only instead of being

parameter_configuration: dict[str, ParameterConfig] = {}

And thus making the it redundant.