Boost: Remove URL override from image guide

[haqadn]

Proposed changes:

• Remove the URL based override from image guide. It was already currently broken and a bit risky in terms of security.

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

No

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

No

Testing instructions:

The image guide functionality should work.

[github-actions]

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