remove unused BACKUP_MULTI_USER_ENABLED

This was added in backup multi-user development but the usage was
removed as backup is de-activated by default for non-system users
and has to be activated on a per user basis.

Bug: 137528046
Bug: 136738613

Test: atest -v CtsBackupHostTestCases
Test: atest -v CtsBackupTestCases
Change-Id: I9475500ea217cad0548f7740ad3580e478fefa9f