Remove Universal Analytics code

[martynmjones]

Changes proposed in this Pull Request:

Removes all existing Universal Analytics code from the extension.

Detailed test instructions:

1. Review the changes and deleted code
2. Run some very basic smoke tests to confirm no errors are triggered by the changes

Additional details:

In addition to removing the UA code, some tracking was moved from class-wc-google-analytics.php c566ea7 into class-wc-google-gtag-js.php 2ee2fad. The tracking itself doesn't need testing here as it will be completely reworked in a follow up PR.

Changelog entry

Remove - Universal Analytics code

[tomalec]

As we removed the "Enhances e-Commerce" section, I think we should move its options to the main section (remove 'class' => 'enhanced-setting').
Currently, it seems like they are a part of "Cross Domain Tracking"

[martynmjones]

Many thanks for the review @tomalec! I've addressed the questions and points you raised so this is ready for a second look 👀.

[tomalec]

Thanks for the changes and clarifications, LGTM