Remove redundant dbus USE variable

CL:455269 adds a USE variable for dbus to the common.gypi file in platform2.
This makes the USE_dbus variable in this file redundant.  Remove it.

BUG=none
TEST='FEATURES=test emerge-$BOARD update_engine' still works
CQ-DEPEND=CL:455269, CL:457783

Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/457871


Commit-Ready: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

(cherry picked from commit 8cc601b7)

Change-Id: Ifff9533fd579ea0e5a6a3d752623605a8c10b541