Cleaning up code style & indentation.

1. Objective

Mainly to cleanup code after a long work of adding in and removing out partial of code here and there.

2. Description of change

For those /includes/gateway/class-omise-payment-*.php classes. Only removing 1 indent.

Back to the PR #153: "Code cleaning for payment method classes", the outer function layer has been removed. However, because I don't want to create an unnecessary LOC at that time, so I leave all the code as it is without moving its indent up 1 layer.

So for this time, just to remove the remaining indentation for the following files:

• includes/gateway/class-omise-payment-alipay.php
• includes/gateway/class-omise-payment-billpayment-tesco.php
• includes/gateway/class-omise-payment-creditcard.php
• includes/gateway/class-omise-payment-installment.php
• includes/gateway/class-omise-payment-internetbanking.php
• includes/gateway/class-omise-payment-truemoney.php

Update code style at /includes/gateway/class-omise-payment.php Removing unnecessary long pyramid layer of indent. Grouping it to make it easier to read Before

protected function invalid_order( $order_id ) {
wc_add_notice(
    sprintf(
        wp_kses(
            __( 'We have been unable to process your payment.<br/>Please note that you\'ve done nothing wrong - this is likely an issue with our store.<br/><br/>Feel free to try submitting your order again, or report this problem to our support team (Your temporary order id is \'%s\')', 'omise' ),
            array(
                'br' => array()
            )
        ),
        $order_id
    ),
    'error'
);
}

protected function invalid_order( $order_id ) {
$message = wp_kses( __(
    'We have been unable to process your payment.<br/>
     Please note that you\'ve done nothing wrong - this is likely an issue with our store.<br/>
     <br/>
     Feel free to try submitting your order again, or report this problem to our support team (Your temporary order id is \'%s\')',
    'omise'
), array( 'br' => array() ) );

wc_add_notice( sprintf( $message, $order_id ), 'error' );
}

Removing a local $order variable, accessing Order object from a class's property instead (make sure that the entire of payment processing is using and manipulating the same Order Object).

3. Quality assurance

🔧 Environments:

WooCommerce: v4.3.1
WordPress: v5.4.2
PHP version: 7.3.3

✏️ Details:

To test invalid_order error message (https://github.com/omise/omise-woocommerce/pull/182/files#diff-b2a67ad6a97418df7dddcefcfc2a5319R401), you will need to modify Omise_Payment::load_order( $order ); code. Right at the first line after an order id has been passed through the method's argument:
```
public function load_order( $order ) {
$order = 'x';
// ...
}
```
To test payment_failed error message (https://github.com/omise/omise-woocommerce/pull/182/files#diff-b2a67ad6a97418df7dddcefcfc2a5319R407), you may place an order with Credit Card payment method using a failed-test-card. (Insufficient fund error card: 4111 1111 1114 0011)