Fix: Make Non-Static Method Not Static; Globalize WP_Term_Order

[ggwicz]

Summary

1. Make Non-Static Method Not Static

• The WP_Term_Order::term_order_add_form_field() method is declared as a static method, but only ever used in a non-static object context.
• This PR removes the unnecessary static method from that method declaration.

2. Globalize WP_Term_Order

• WP Term Order has hook callback methods that cannot be unhooked via remove_action(). This is bad practice for a WordPress plugin or theme, and should be fixed.
• A few different fixes are possible; one example is to implement a singleton design pattern for the WP_Term_Order class.
• But to keep things simple, I've just made the single existing object (created on an init hook callback) a global variable: global $wp_term_order.
• This global can then be referenced by remove_action() calls, without otherwise affecting any existing plugin functionality.