[FIX] onboarding: dark variables not loaded

This commit fixes an issue about the `onboarding.variables.dark.scss`
file not being taken into account inside the manifest.

Since saas-16.4, there is an issue with the onboarding files.
If you turn on the dark mode, the `.variables.dark` overrides are not
taken into account, but instead display the light mode variables.

This causes readability issue about the text being too dark.

This seems to happen because the dark mode file is already loaded
with the light one.

task-3201038

closes odoo/odoo#140090

X-original-commit: 9e1e72bfff1a8f27513dbb2a2a2aae0213bc78ed
Signed-off-by: Pierre Paridans (app) <app@odoo.com>
Signed-off-by: Chrysanthe Gomrée (chgo) <chgo@odoo.com>

addons/onboarding/__manifest__.py

@@ -21,6 +21,7 @@ This module allows to manage onboardings and their progress
     'assets': {
         'web.assets_backend': [
             'onboarding/static/src/**/*',
+            ("remove", "onboarding/static/src/scss/onboarding.variables.dark.scss"),
         ],
         "web.dark_mode_variables": [
             ('before', 'onboarding/static/src/scss/onboarding.variables.scss', 'onboarding/static/src/scss/onboarding.variables.dark.scss'),