gnu: tryton.scm: Remove unused native inputs.

gnu: tryton.scm: Remove unused native inputs.

Python is already provided by the python build-system,
thus there is not need for listing it a native input.

* gnu/packages/tryton.scm (python-trytond-account,
  python-trytond-account-invoice, python-trytond-account-invoice-stock,
  python-trytond-account-product, python-trytond-analytic-account,
  python-trytond-company, python-trytond-country, python-trytond-currency,
  python-trytond-party, python-trytond-product, python-trytond-purchase,
  python-trytond-purchase-request, python-trytond-stock,
  python-trytond-stock-lot, python-trytond-stock-supply)[native-inputs]:
  Remove python.