Remove anything related to contact_info deprecated field

[nicholaspcr]

Summary

This issue serves as a tracker, being referenced in code to ensure that on a major bump the API definitions, methods and tests will be removed from the codebase.

References:

• Replace ContactInfo with UserIdentifiers as Administrative/Technical/Security Contacts #4930
• Remove contact_info completely from the email validation process #6567
• Remove storage of the contact_info in most entities and make it read-only on User storage #6515

Current Situation

Currently the field is deprecated and with the changes made on the referenced issues, the field itself will be used as a read-only field of the entities.

Desired Situation

When doing a major/bump release we should remove the unnecessary API routes, methods and tests.

Code of Conduct

• I agree to follow TTN's Community Code of Conduct.