Remove duplicate viewport and title element hooks

[jeremyfelt]

Description

The hooks to inject gutenberg_viewport_meta_tag and gutenberg_render_title_tag into wp_head were shipped as part of WordPress 5.8. Having them in the Gutenberg plugin and in WordPress core results in duplicate <title> and duplicate <meta name="viewport" tags in markup.

See WordPress/wordpress-develop@ea49625

How has this been tested?

1. View source on the front-end before the change.
2. View source on the front-end after the change.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• n/a I've tested my changes with keyboard and screen readers.
• n/a My code has proper inline documentation.
• n/a I've included developer documentation if appropriate.
• n/a I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[jeremyfelt]

I guess this may actually not be the best way to remove the duplication if the Gutenberg plugin is still supporting WP pre-5.8. If that is the case, it may be better to add some function_exists() calls.

[Mamaduka]

I guess this may actually not be the best way to remove the duplication if the Gutenberg plugin is still supporting WP pre-5.8. If that is the case, it may be better to add some function_exists() calls.

Yes, we have to support WP < 5.8 versions.

The current method for similar cases is to remove WP Core filters and only use hooks from the plugin. See: #33233 (comment)

[jeremyfelt]

This was fixed in #36133 to fix #34106, which was somewhat of a duplicate of this issue.