[Gutenberg] Hide audio block on free sites

[ceyhun]

Hide audio block on free sites.

gb-mobile PR: wordpress-mobile/gutenberg-mobile#3129

Testing

1. Switch to a free/simple site
2. Open editor
3. Tap block inserter
4. Audio Block should not be in the list

1. Switch to a paid/self-hosted site
2. Open editor
3. Tap block inserter
4. Audio Block should be in the list

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[jd-alexander]

Thanks for these changes @ceyhun Tested using the following flows:

1. WPCom simple site with a free plan. - Audio block is not shown in the inserter.
2. WPCom simple site using a paid plan - Audio block is shown in the inserter.
3. Self-hosted site - Audio block is shown in the inserter.