Removed cube_api_proxy flag

What does this MR do and why?

• To reduce the number of feature flags used for the internal preview being built by groupproduct analytics, we're removing one flag and merging its uses in to another.

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390700 (closed)