greybus: drop some unnecessary headers

There's no need to include anything other than "greybus.h" in
"connection.c".  Same thing in "core.c" and "manifest.c" and
"svc.c".  Some files need headers included, but most come along
with "greybus.h".

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>