Use `Keys::DestroyService` for deleting an SSH key when a user deletes a key via the API

What does this MR do?

Contributes to #220127 (closed)

We are not adding AuditEvent creation to Keys::DestroyService now as the refactoring of audit_events table is going on, and we have temporarily halted adding of new audit event types till that is complete.

This change is just a tech-debt fix, so that wherever SSH keys are destroyed across the codebase, Keys::DestroyService is used.

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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
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