You can null the file easily, please follow the steps below:
Got to application/controllers/Install.php from control panel
Then find the line
function validate_purchase_code() {
$purchase_code = $this->input->post('purchase_code');
$validation_response = $this->crud_model->curl_request($purchase_code);
if ($validation_response == true)
Change 'true' to 'false' and save the file
Then start the installation process.
I have attached the file for your reference.