Remove alert endpoint dropdown user callout

What does this MR do?

A small MR to remove the unused user call out methods from the ~"Category:Alert Management" feature service dropdowns.

This call out was behind feature flag (#225496 (closed)) hence not changelog entry.

We break this out as we need to remove callout values from the database.

Will resolve #224668 (closed).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - it was behind feature flag - see #225496 (closed)
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team