[FIX] web_editor: allow to reset background color

Before this commit:

When copy-pasting from Discord into Notes, get a text with color and background
color. But when clicking on the trash icon inside the toolbar to remove the
background color, it doesn't remove the background color.

After this commit:

Allowed to remove background color from text using trash icon.

Task-2889682

closes odoo/odoo#96501

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>