Remove json2

[johnbillion]

In WordPress 6.9, the json2 library was no-oped as it's no longer needed by any browser. This was done as part of a larger change to remove support for old IE conditional syntax. See https://core.trac.wordpress.org/ticket/63821 and WordPress/wordpress-develop@49d1ded.

This PR removes json2 from the Freemius SDK. This should be a safe change to make because:

• If a Freemius-powered plugin depends on json2 then it'll still get enqueued via the normal script dependency system in WordPress. The json2 script is still registered, it's just no longer enqueued manually by the Freemius SDK.
• The json2 library provides polyfills for JSON.stringify() and JSON.parse() by pushing its implementations onto the global JSON object. No browsers need this back compat, and there is nothing provided in json2 which is not provided by JSON in the browser.

Fixes #840.