Revert cleanup of flag "contextual_search_prevent_self_capture"

Reason for revert: Instructions from the bot were wrong. Cleaning
up the flag does not actually work and breaks tests. Good job, bot.

Reverted changes: /q/submissionid:33073277-cspsc-cleanup

Bug: 412923541
Bug: 410533482
Change-Id: I723ffd446a3c4ab866324e7e96732ea08acf3961