[FIX] website: remove bg image support for columns in s_cover snippet
[FIX] website: remove bg image support for columns in s_cover snippet

The snippet section can have a color or a background. Previous commit
added the functionnality for its innercolumn too... but having a bg
image on top of another one does not make sense, so only keep the
color option for columns.

hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo