Custom CSS: remove outdated link from admin menu

[monsieur-z]

Fixes https://github.com/Automattic/vulcan/issues/479

Proposed changes:

The Custom CSS module was removed from the Jetpack plugin but, in the admin menu, there's a lingering link (labelled Additional CSS) to the Custom CSS section in the Jetpack settings, that no longer exists. This PR removes the link.

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Project thread: pfwV0U-4M-p2

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Spin up a test site with this branch
• Install a classic theme, such as Twenty-Eleven
• Notice the Appearance menu has no Additional CSS item