Removes usage_data_diff_searches flag definition

What does this MR do and why?

We'd previously attempted to remove this flag in !84320 (merged), however due to the dynamic methods used to track front end statistics, we had to leave the flag extant within the system (even if never used.) That system of tracking has been updated to remove this requirement, so we can finally remove the last vestiges of this flag.

• Removing flag definition and call sites: !84320 (merged)
• Re-adding flag definition: !86603 (merged)
• Confirmation that we can now remove this flag: #384858 (comment 1203095452)

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Related to #384858 (closed)