Remove ability to add a template part in the post editor

[annezazu]

Description

Remove ability to add a template part in the post editor, including copy and pasting. This was previous in place https://github.com/WordPress/gutenberg/pull/37157 but seems to now be allowed to add once more. I can replicate this with 6.1 beta 1.

Step-by-step reproduction instructions

1. Use GB 14.1.1
2. Open post editor
3. Start to add a template part block however you'd like.
4. Notice that you can.
5. Deactivate Gutenberg
6. Open post editor
7. Try to add a template part block and notice you can't

Separately, notice that you can copy/paste a template part block from the template editor into a post editor, with or without Gutenberg installed, causing multi entity saving to appear in the post editor.

Screenshots, screen recording, code snippet

https://user-images.githubusercontent.com/26996883/191822627-f0276537-8fb7-4b26-9b65-b0807d471619.mov

Environment info

• WordPress 6.1 beta 1
• Gutenberg 14.1.1 both activate and deactivated
• TT2

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes