remove reference to infer_spectral_model method

Author: cwhanse

docs/sphinx/source/reference/modelchain.rst

@@ -112,7 +112,6 @@ on the information in the associated :py:class:`~pvsystem.PVSystem` object.
    modelchain.ModelChain.infer_dc_model
    modelchain.ModelChain.infer_ac_model
    modelchain.ModelChain.infer_aoi_model
-   modelchain.ModelChain.infer_spectral_model
    modelchain.ModelChain.infer_temperature_model
    modelchain.ModelChain.infer_losses_model