devices: usb: remove unused imports in tests
devices: usb: remove unused imports in tests

Clean up warnings when building tests.

BUG=None
TEST=cargo test -p devices; FEATURES=test emerge-nami crosvm

Change-Id: I0efe723fee37c822a6bdd4a88766b37d559833d8
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1560092
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: David Tolnay <1480464b87745a0822a0e8b7545ca981d8e7352e@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm