Fix two tests in test_units.py module

In these two tests in the module test_units.py:

def test_units_dimensionless_imported_function(meta_model, model_kwargs)
def test_units_dimensionless_imported_function_error(meta_model, model_kwargs)

the variable freq = 1 [Hz] is defined but not used. The variable should be either removed or used instead of the constant 1 [Hz].