Boost: Make sure source provider URLs are unique

[dilirity]

Proposed changes:

• Remove core_front_page provider and code removing it. We added this when working on Cornerstone Pages, and the logic was behind a feature flag. There's no longer a feature flag, so it's okay to remove the source provider and the code that was previously removing it.
• Update critical css UI to consider cornerstone pages as important and show an error if one of them fails. core_front_page is no longer an important page as it no longer exists as a provider. This partially addresses Boost: Should C.CSS generation continue if an important provider fails #40391.
• Remove WP Core Provider URLs from list, if they are present in Cornerstone Pages list;
• Remove possible duplicate blog page, if it was in the cornerstone pages list when a static page was not set, but a blog page was set.
• Fix bug that made "Load default pages" button clickable after loading defaults and saving. This happens when Yoast had the home page as a cornerstone page and it was also in Boost's cornerstone pages list.

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

n/a

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

no

Testing instructions:

• Setup Boost (free);
• You can use Boost companion, to easily check the list of source providers by opening http://jetpack-boost.test/wp-json/jetpack-boost/v1/list-source-providers
• Add some pages, set a static page, no blog page, add cornerstone pages. In other words - play around with the cornerstone pages and see if the list of source providers behaves correctly.

[github-actions]

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[haqadn]

Skipping from this beta as we still need to finish the discussion about what is an important provider for critical CSS.