Remove a now-unneeded ES rake task

What does this MR do?

Removes a rare elasticsearch migration rake task that is no longer necessary.

We expect people to run these things within the same major release. Now %12.0 is released, it can be removed.

Does this MR meet the acceptance criteria?

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