Revert "LegacyUSBDeviceManager: reintroduce persist.service.adb.enable"

This reverts commit 60337aafd76a6fd8de477e280619bc0335603252.
Revert "LegacyUsbDeviceManager: register AdbSettingsObserver unconditionally"

This reverts commit bbba92693b2c10f55a64fea227e211fe91f659cc.