[FIX] *: fix duplicate palette crashing render in theme tab

*: theme_anelusia, theme_avantgarde, theme_bookstore, theme_buzzy,
   theme_real_estate, theme_treehouse

Since website refactor [1], a silent error would occur while rendering
the theme tab, because of the use of `t-foreach`, leading to nothing
being displayed depending of the theme.

This was caused by a duplicate key in the
`$o-selected-color-palettes-names` scss variable, added by some theme,
which would make the `t-foreach` crash in `website.ThemeColorsOption`.

Steps to reproduce:
- Create a website using any of the above themes, through the wizard
- Enter edit mode
- Click on theme tab
- Nothing is rendered

[1]: https://github.com/odoo/odoo/commit/9fe45e2b7ddbbfd0445ffe25a859e67a316d02b2

opw-5020952
opw-5040694

closes odoo/design-themes#1141

X-original-commit: d53ae16a85
Related: odoo/odoo#225809
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Signed-off-by: Augustin Dupin (duau) <duau@odoo.com>

theme_avantgarde/static/src/scss/primary_variables.scss

@@ -186,8 +186,6 @@ $o-website-values-palettes: (
     ),
 );
 
-$o-selected-color-palettes-names: append($o-selected-color-palettes-names, 'default-15');
-
 $o-color-palettes-compatibility-indexes: (
     1: 'avantgarde-1',
     2: 'avantgarde-2',