Feature/3644 remove rest batch infrastructure

[ivankruchkoff]

Summary

Addresses issue #3644

Relevant technical choices

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

[eugene-manuilov]

Thanks, @ivankruchkoff. Mostly looks good, but there are a few things that we need to address before we can merge this PR. Please, take a look.

[eugene-manuilov]

Thanks, @ivankruchkoff. It looks good to me.