Fix rollback of migration that adds temporary index for container scanning findings

What does this MR do?

Just in case a self hosted instance has to rollback this migration we don't try to drop wrong index.

!39962 (comment 425208906)

Conformity

• Changelog entry
• Documentation (if required)
• Code review 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