Cleanup use new backup eligibility rules feature flag

This flag has been enabled since 2021, we can safely remove it now.

Bug: 432673356
Test: atest ./services/tests/mockingservicestests/src/com/android/server/backup/UserBackupManagerServiceTest.java
Test: atest ./services/robotests/backup/src/com/android/server/backup/UserBackupManagerServiceTest.java
Flag: EXEMPT PURE_REFACTOR
Change-Id: I546273ac1c07870adc27d6e910fc5fbd7036d000