remove TaxExemptionReason from test files

several UBL test files contain a cac:AllowanceCharge/cac:TaxCategory/cbc:TaxExemptionReason element, which throws UBL-CR-481, thus should be removed. The same content is provided within cac:AllowanceCharge/cbc:AllowanceChargeReason (= BT-97), therefore, visu output is not affected by removal.