[18.0][FIX] l10n_it_edi_extension: fix for unexpected keywoard 'vat_only'

if l10n_it_edi_witholding is installed and it is loaded after l10n_it_edi_extension,
when _l10n_it_edi_get_extra_info on l10n_it_edi_withholding/models/account_move.py calls
_l10n_it_edi_search_tax_for_import , it calls the function of l10n_it_edi_extension
that does not have the vat_only keyword

Author: micheledic