remove from system_models.rst

Author: RDaxini

docs/sphinx/source/reference/pv_modeling/system_models.rst

@@ -13,7 +13,6 @@ Sandia array performance model (SAPM)
    pvsystem.sapm
    pvsystem.sapm_effective_irradiance
    pvsystem.sapm_spectral_loss
-   spectrum.spectral_factor_sapm
    inverter.sandia
    temperature.sapm_cell