[FIX] web_editor: fix image removal inside image gallery

Since we use Promise instead of jQuery deferred, the onRemove method
class was no longer triggered before the actual removal of the elements.

Original fix with https://github.com/odoo/odoo/pull/43918


This PR is the same with the async/await form, not breaking the history
of half of the removeSnippet method's lines.

closes odoo/odoo#44900

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: fja-odoo <fja@odoo.com>