Remove ACF4 code

[enricobattocchi]

Summary

This PR can be summarized in the following changelog entry:

• Remove all ACF 4-specific code.

Relevant technical choices:

• the JS test is not actually working. The ACF4 tests have been removed from there anyway.

Test instructions

This PR can be tested by following these steps:

• Install ACF4, the latest release for the 4.* branch can be downloaded here
• Install Yoast SEO
• checkout this branch and activate
• create a new group of custom fields for posts in ACF, sticking to just the ones that allow you to add texts (they should be Textbox, Textarea, Email, WYSIWYG)
• edit a post, and see that adding text in those fields doesn't change the number of words counted by the SEO analysis
• see alsto that everything else is working and there are no fatal errors in the console that break the behaviour of the block editor, or of Yoast SEO.
• perform the same test with Classic Editor.
• delete ACF4 and installa ACF5 Free (you can do it from Plugins→Add new, otherwise download from here)
• create a new group of custom fields for posts in ACF, you can import the ZIP that you can find in this PR, rememeber that Repeater and Flexible types won't be working.
• test that the analysis is working (follow the suggestions to test P3-147 Fix refresh analysis #275) and there are no console errors.
• perform the same test with Classic Editor.
• Remove ACF5 and install ACF 5 Pro, this would enable Repeater and Flexible fields if you are using the zip from above
• test that the analysis is working (follow the suggestions to test P3-147 Fix refresh analysis #275) and there are no console errors.
• perform the same test with Classic Editor.

Fixes [P3-167]

[jcomack]

CR done 👍

[JesserH]

Acceptance ✅