composeinfo: fix arch kwarg handling in get_arches()
composeinfo: fix arch kwarg handling in get_arches()

Prior to this change, if a caller specified an arch parameter to
get_arches(), productmd would crash with an AttributeError, because were
checking the arches member on a VariantBase object that probably does
not have that member defined.

Remove the check completely, since this probably never worked.

release-engineering/productmd,lubomir/productmd,release-engineering/productmd,lubomir/productmd