Revert "adb: fix deadlock between transport_unref and usb_close."

This reverts commit 7e197ef8.

The mutex lock in transport_unref hides a race that seems otherwise
hard to fix. Specifically, there's no synchronization between acquiring
a transport and attaching it to an asocket*, leading to badness if the
transport is closed in between the two operations.

Fix the original problem the reverted patch addressed by manually
unlocking before calling unregister_usb_transport.

Bug: http://b/65419665
Bug: 64709603 (presubmit balking at the line above)
Test: python test_device.py
Change-Id: I0ed0044129b1671b2c5dd1b9fa2e70a9b4475dc5
(cherry picked from commit e48ecce6)