[FIX] web_editor: make monetary field unremovable

Before this commit, the span within monetary field could wrongly be
removed. Python code that parse the value (see `from_html` of
`ir.qweb.field.monetary`) expect to have that span available.

This commit make that span being unremovable.

opw-3090775

closes odoo/odoo#107421

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