[FIX] web_editor: fix small memory leak of destroyed snippet editors

In some complex cases, some snippet editors were destroyed but were
still referenced in memory.

E.g.

1) Reorder a snippet gallery image
2) The code is implemented in such a way that reordering create a new
   wrapping column for each image and moves the images in them.
3) This result in having the snippet editor linked to the old columns
   to be explicitely destroyed and removed from memory, but not the ones
   linked to the images as they are still in the DOM (just moved).
4) ... but the snippet editors linked to the images are children of
   the snippet editors linked to the columns. They are thus implicitely
   destroyed... but not removed from memory.

closes odoo/odoo#54643

X-original-commit: e4bf37c7fc892cdba57f0acac8e10f5698184e67
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>