Remove deprecated `sopresto` code

[MaxwellGarceau]

Is your enhancement related to a problem? Please describe.

The Sopresto integration is no longer part of the Mailchimp WP plugin and should be removed to prevent confusion moving forward.

• Verify that the Sopresto code is no longer in use
• Remove all references to "sopresto"
  • mailchimp_sf_migrate_sopresto - no references in the codebase

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Split from issue #62.
Split from PR #85 in the comments

Designs

No response

Describe alternatives you've considered

No response

Code of Conduct

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