Remove flag for persisting restore-to-rare apps list.

This feature has been rolled out in A16.

Fixes: 410454106
Test: atest UsageStatsDatabaseTest
Test: atest AppIdleHistoryTests
Flag: EXEMPT removing com.android.server.usage.persist_restore_to_rare_apps_list
Change-Id: I641467b7d683cce1160b54f7aad5bedc3f24194f