adb: libusb: don't try to delete a usb_handle twice.

Previously, we would attempt to delete a usb_handle in both
device_disconnected and usb_close. If the one in device_disconnected
happened to happen first, usb_close would abort when it failed to find
the handle it was supposed to own.

Bug: http://b/62020217
Test: unplugging device on darwin
Change-Id: I6c6bf61bf89a4d9a23458c00b457080d3d6cc744