[FIX] web_editor: fix .img-fluid for non-Outlook

Since flexbox is not well supported in e-mails, the Bootstrap grids
don't stack. This means that the .img-fluid images have to adapt to
columns that are smaller than what they were designed for. We remove the
dimensions on the images to let .img-fluid images resize percentually.

Part-of: odoo/odoo#125146