Remove security report comparison caching

What does this MR do and why?

This report caching creates a lot of complexity in the report comparison process which creates quite a lot of bugs. Since !201475 (merged) was implemented, the frontend now polls a different graphql endpoint to determine if the security MR widget is ready. This endpoint uses a cheap database query. Thanks to this, we can remove the caching code as we are no longer polling the expensive endpoint.

References

Relates to: #593774 (closed), #585505 (closed)

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