Character Encoding: Remove meta tag after injecting Form

[n7studios]

Summary

This previous PR resolved an issue with character encoding when injecting a Form after the nth element (heading, paragraph, image etc).

This resulted in an errant meta tag being left behind in the Post's content:

This PR resolves by removing the meta tag.

Testing

Updated tests to confirm meta tag removal when injecting a Form after an element.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)