greybus: build: remove Android makefile

There is no standard way of building a kernel from source in
Android.  Each device/SoC can (and do) implement it in their own way.
To that end, let's remove this makefile and let each device define
how they want to build the modules.

Signed-off-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>