refactor: replacing fee subtracting mechanism in setup_refund

[wei3erHase]

cd noir-projects/noir-contracts/
./bootstrap.sh compile token_contract
./boostrap.sh test token_contract

Fails with:

[token_contract] Testing test::refunds::setup_refund_insufficient_funded_amount... FAIL

error: Test failed with the wrong message. 
Expected: Balance too low 
Got: Failed calling external resolver. ErrorObject { code: InvalidRequest, message: "Assertion failed: max fee not enough to cover tx fee 'max_fee >= tx_fee'", data: None }