Remove cached_mr_widget feature flag

What does this MR do and why?

As discussed in #332966 (comment 683932353), we are no longer pursuing the approach. Because of that, we're removing the feature flag.

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 #330803 (closed)