Remove BnServiceManager.

As far as I can tell (e.g. https://goo.gl/nxpFBZ from 2009),
BnServiceManager is unused. It's only relevant for
libbinder-based C++ daemons implementing the
IServiceManager.aidl interface. service_manager.c rolls its
own binder code instead.

Change-Id: I837a897d8a201b04bf3ff65c87b0d35d10196199