Remove enable_restricted_bucket flag.

The RESTRICTED bucket has been enabled for a few releases and we have no
intention of disabling it by itself at this point.

Bug: 273564840
Test: atest AppStandbyControllerTests
Test: atest CtsJobSchedulerTestCases:ConnectivityConstraintTest
Test: atest CtsJobSchedulerTestCases:JobThrottlingTest
Test: atest CtsUsageStatsTestCases:UsageStatsTest
Test: atest SettingsBackupTest
Change-Id: I029832fb505a46dc55d966cbb449f52b0ccb9989