Cleanup backup_service_security_log_event_enabled

Flag was rolled out in V. Keeping flag definition since it is exported

Bug: 335663055
Test: TH
Flag: EXEMPT flag cleanup
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1143776ca208c9ff843b17ea48c0da6f383963da)
Merged-In: I0d8074d3fb82cbcbe68b2436261a1741f77f9d9f
Change-Id: I0d8074d3fb82cbcbe68b2436261a1741f77f9d9f