Removed ionic model used in tutorial "Adjusting wavelength to experimental data"

In the tutorial entitled "Adjusting wavelength to experimental data", the ionic model labeled GTT2_fast is used. Yet, this model was dropped in version 9.0 of openCARP.

In the experiments repository, some results obtained with this model are provided, and the label appears in the parameter files tuning_wavelength_b10_cable.par and tuning_wavelength_state_cable.par