[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#44963

X-original-commit: 37941159c82d487ff172f0e6b27028f23ab32810
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: fja-odoo <fja@odoo.com>