Remove starter site launch onboarding promo notice

[harshitarora-in]

Goal / Acceptance Criteria

This onboarding notice was introduced in PR #465 (starter-sites-onboarding-notice) to promote newly launched starter sites.

The campaign has now ended and the notice should be removed.

Requirements:

• Review PR starter-sites-onboarding-notice #465 and identify all code added specifically for the onboarding promo notice.
• Remove the onboarding promo notice from the Site List onboarding step.
• Remove any related styling, localization strings, AJAX handlers, option storage, and helper functions that are exclusively used by this notice.
• If any code appears reusable or is referenced elsewhere, leave it in place and document it in the PR.

Validation:

• The onboarding notice no longer appears.
• The onboarding flow continues to function normally.
• No onboarding navigation or redirect behavior is affected.
• No console errors, PHP warnings, or failed tests are introduced.
• All changes are limited to functionality introduced for the promo notice.

Please create a PR summarizing:

• Files modified
• Code removed
• Any intentionally retained code and why
• Any areas requiring developer review

Constraints / Do Not Touch

Do not modify unrelated onboarding functionality.

Do not change:

• Site import flow
• Template library functionality
• Licensing checks
• Upgrade prompts unrelated to this notice
• Onboarding navigation or redirect logic

Keep changes focused only on the onboarding promo notice introduced in PR #465.

Relevant Files (if known)

No response

Tests to Run

Default:

• composer run lint
• composer run phpstan
• phpunit

When JS files are touched:

• yarn run lint
• yarn run build

When UI/E2E flows are touched:

• npm run ci:e2e