[FIX] web_editor: unobserve background grid z-index

Minor fix: the background grid is not observed in the first place,
setting a z-index on it should not be observed either.
Noticed when fixing `automaticStep(Un)active()` at [1].

[1]: https://github.com/odoo/odoo/commit/c2d952689c97add74a372c90aa18787094d5d93a

Related to task-3576046

closes odoo/odoo#147970

X-original-commit: 901ed6d8c6c6bdb8aff8e9366d63bba59051019a
Signed-off-by: Guillaume Dieleman (gdi) <gdi@odoo.com>

addons/web_editor/static/src/js/editor/snippets.options.js

@@ -4600,8 +4600,8 @@ registry.sizing = SnippetOptionWidget.extend({
             if (rowEl.classList.contains("o_grid_mode") && !isMobile) {
                 self.options.wysiwyg.odooEditor.observerUnactive('displayBackgroundGrid');
                 backgroundGridEl = gridUtils._addBackgroundGrid(rowEl, 0);
-                self.options.wysiwyg.odooEditor.observerActive('displayBackgroundGrid');
                 gridUtils._setElementToMaxZindex(backgroundGridEl, rowEl);
+                self.options.wysiwyg.odooEditor.observerActive('displayBackgroundGrid');
             }
 
             // For loop to handle the cases where it is ne, nw, se or sw. Since