[fix] categories can't be removed from UI (categories_as_tabs)

When using ``use_default_settings: true``, removing default categories from
settings.yml will not remove them from the UI.

The value ``categories_as_tabs`` is a dictionary type (a4c2cfb8) and dictionary
types are merged additive by ``settings_loader.update_settings()``.

This patch replaces the default ``categories_as_tabs`` by the one from the
``user_settings``.

Related: https://github.com/searxng/searxng/issues/1019#issuecomment-1193145654


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>