Delete a Vulnerability-Issue link API call

What does this MR do?

Implements the API endpoint to delete a link between issue and Vulnerability for the Issues associated with Vulnerabilities functionality for MVC Standalone Vulnerability objects. This is only a part of this functionality, other API operations are delivered as separate MRs (example: getting a list of related issues for a vulnerability).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry | This is backstage issue so no changelog. Changelog entry will be added for the entire feature when the feature flag is removed.
• [-] Documentation (if required) | No documentation because it was decided that the docs for all of the APIs related to MVC Standalone Vulnerability objects should be delivered in form of documentation stubs.
• 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

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