[FIX] web_editor: align icon to the right

When replacing an image by a pictogram, the classes of the previous media
are copied to the new icon's `<span>`. This potentially includes the
`w-100` class which prevents the alignment from behaving correctly.

This commit removes the `w-100` class if it exists - in addition to
removing the inline `width` attribute which was done before.

Steps to reproduce:
- Drop a "Media List" block
- Replace first image by pictogram.
- Align icon to the right.
=> Icon was displayed aligned to the left.

task-2829971 (was task-2729177)

Part-of: odoo/odoo#89010