Plugin: Remove redundant core compatibility from plugin

[aduth]

Related: #11015

This pull request seeks to remove (deprecated) logic in lib/compat.php assumed to be present in core.

The primary core tracking issue is:

• https://core.trac.wordpress.org/ticket/45113

The only remaining compatibility is that of gutenberg_show_privacy_policy_help_text. I could not find an equivalent core compatibility. The one here assumes wrongly that Gutenberg will upgrade the notices, which was removed as part of #12444 . This requires more work. In the meantime, the code in lib/compat.php has been left as-is.

Testing instructions:

Verify there are no regressions in the loading of the editor.

[aduth]

The only remaining compatibility is that of gutenberg_show_privacy_policy_help_text. I could not find an equivalent core compatibility. The one here assumes wrongly that Gutenberg will upgrade the notices, which was removed as part of #12444 . This requires more work. In the meantime, the code in lib/compat.php has been left as-is.

In doing some further investigation and in discussing with @desrosj , it's true that this compatibility does not exist. However, the logic lingering here in lib/compat.php does nothing to help, and thus I'm inclined to defer to a core revision to reimplement the notice for the block editor.

To this end, I've created a Trac ticket: https://core.trac.wordpress.org/ticket/46098

As such, I'll plan to include here a deprecation of gutenberg_show_privacy_policy_help_text.