Revert "cashew: do not delete DBus::ObjectProxy objects from D-Bus callbacks"

This reverts commit 35386a9949355d796f2b195b0577e5a39fe72477.

Reverting this CL in preparation for a more comprehensive CL that will
also address the DBus::ObjectProxy creation case, which can also trigger
a deadlock if done in a D-Bus callback.

Change-Id: Ic60a1eb517b1cd76997aa3f8e00b42c44eeb018c

BUG=chromium-os:8892
TEST=emerge cashew

Review URL: http://codereview.chromium.org/5284004