Remove ACF4 code - Githubissues

Summary

This PR can be summarized in the following changelog entry:

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

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 #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 #275) and there are no console errors.
perform the same test with Classic Editor.

Fixes [P3-167]