CE destroy repo mirrors

What does this MR do?

CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10355

destroy repo mirrors when the delete button is clicked.

it is important to destroy instead of disable because it opaquely leaves sensitive data in the DB that could worsen a breach.

Use _destroy nested attribute instead of enabled for push mirrors.

Call remove_import_data after saving a project if its pull mirror is disabled.

Does this MR meet the acceptance criteria?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54574

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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