Revert "adb: turn on libusb by default."

This reverts commit f2f0b318.

libusb seems to be causing flakiness on some machines. Disable it for
now.

Bug: http://b/62962248
Test: python test_device.py (with DeviceOfflineTest commented out)
Change-Id: Ia9de78ab772c22574cf5ca7facb78f22af6d7a71
(cherry picked from commit 969110ca)