[TASK] Remove Invalid Shipping Tax Calculation Algo

The 2nd option "Use Tax Depending on Product Values" did a wrong calculation so I removed it for now. To make that visible to old users I created a flag to store if the old config is still wrong or has been fixed (might be necessary for future upgrade where the old config value '2' is used again) and display one message after login plus a static notification. I also fixed a typo and a bug.