[FIX] web_editor: remove old values in the color picker widget previews

This [first commit] removes the addition and removal of classes on the
color picker preview. So far so good, then this [other commit] puts back
the class addition but forgets the removal so the preview of the color
picker widget kept the old values when they were `bg-*` which could lead
to a bad preview of the selected color. This commit fixes that by
removing the old values when a new value is set.
Steps to reproduce the bug fixed by this commit:
- Go to the website app
- Edit a page
- Drop a Picture block
- Change the background color of the block to bg-black for example
- Change again the background color of the block to a custom color

=> The preview of the custom color is not correct, it is still black.

[first commit]: https://github.com/odoo/odoo/commit/212a8bfdd21269b18054200b9e2585e1c95540d6
[other commit]: https://github.com/odoo/odoo/commit/a396f791da94d064d58cce15892e74f45d29b7b7

task-2904507

closes odoo/odoo#101828

X-original-commit: 6da33797798e6112e0dd74a4093f296b6e1b2466
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

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

@@ -1476,6 +1476,8 @@ const ColorpickerUserValueWidget = SelectUserValueWidget.extend({
 
         await this._colorPaletteRenderPromise;
 
+        const classes = weUtils.computeColorClasses(this.colorPalette.getColorNames());
+        this.colorPreviewEl.classList.remove(...classes);
         this.colorPreviewEl.style.removeProperty('background-color');
         this.colorPreviewEl.style.removeProperty('background-image');
         const prefix = this.options.dataAttributes.colorPrefix || 'bg';