buffet: Remove dbus_command_proxy_unittest.cc as irrelevant

Tests have a lot of unrelated dependencies and tests more than they
should.

BUG=brillo:1245, brillo:1263
TEST='FEATURES=test emerge-gizmo buffet'

Change-Id: I33a851543b08ff421cb19fc063a440616170f99e
Reviewed-on: https://chromium-review.googlesource.com/288254
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>