You wanted no tax code or WHT code to appear or you wanted to remove the tax code that was determined automatically by system.
SAP Business ByDesign
In general tax code and WHT tax code are determined automatically based on the tax details maintained in Product master data or Supplier master data.
Hence though you remove again system determines it, thus you cannot make the tax code as empty.
So you can either
Remove the tax details in the Supplier master and product master
Enter a tax code that has zero rate
empty tax code, remove with holding tax code , KBA , SRD-SRM-SI , Supplier Invoicing , AP-POP-PO , Purchase Order , Problem