Multi-Currency: Fix unused parameter phpcs sniffs

[reykjalin]

Part of #8436.

Description

Remove the VariableAnalysis.CodeAnalysis.VariableAnalysis exclude from phpcs.xml.dist and fix the issues reported when running npm run lint:php in the following files:

• includes/multi-currency/FrontendCurrencies.php
• includes/multi-currency/Analytics.php
• includes/multi-currency/Compatibility/WooCommerceProductAddOns.php
• includes/multi-currency/MultiCurrency.php
• includes/multi-currency/Compatibility/WooCommerceSubscriptions.php
• includes/multi-currency/Settings.php
• includes/multi-currency/SettingsOnboardCta.php
• includes/multi-currency/Compatibility/WooCommerceNameYourPrice.php

Suggested fixes:

• When a variable is truly unused, such as in filter functions where a parameter isn’t needed, prepend $_unused_ to the variable name. For example $order → $_unused_order.
  • When a child class implements a function but ignores one of the parameters, use a PHPCS ignore until PHPCS/Psalm: Figure out how to reconcile $_unused_ var name prefixes in child classes with Psalm static analysis #8956 is fixed.
• If a variable is not necessary, remove it.
• When a parent class/interface doesn’t use a parameter but child classes do, use an inline // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable.