Remove unused VSA use_aggregated_backend endpoint

What does this MR do and why?

This MR removes the unused use_aggregated_backend endpoint which was used before 15.0 to switch off the VSA aggregated backend and fall back to the old behaviour.

Since then the aggregated backend was enabled by default and the endpoint is no longer accessible via the frontend so it's safe to remove it. Not an user-facing change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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