invoicing: use simpler condition for Invoice_isAdvanced

bt5/erp5_invoicing/SkinTemplateItem/portal_skins/erp5_invoicing/Invoice_isAdvanced.py

@@ -2,7 +2,5 @@
 If the business template 'erp5_advanced_invoicing' is installed, returns True.
 If it is not, returns False.
 """
-for bt in context.portal_templates.getInstalledBusinessTemplateList():
-  if bt.getTitle() == "erp5_advanced_invoicing":
-    return True
-return False
+return context.getPortalObject().portal_templates.getInstalledBusinessTemplate(
+    'erp5_advanced_invoicing', strict=True) is not None