[FIX] pos_mercury: Fix duplicate xml id

Follow up on https://github.com/odoo/odoo/commit/63f052a40072e3a5487a8fe61c29f29d48807334#r34578119
Due to this https://github.com/odoo/odoo/blob/12.0/addons/pos_mercury/views/pos_config_setting_views.xml#L3

 was being replaced by this view (See manifest for info)

closes odoo/odoo#35471

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>