bus: don't force send-masks on kdbus buses

Right now we always pass KDBUS_ITEM_ATTACH_FLAGS_RECV to
KDBUS_CMD_BUS_MAKE, effectively forcing every bus connection to do the
same during KDBUS_CMD_HELLO. This used to be a workaround to make sure all
metadata is always present. However, we refrained from that approach and
intend to make all metadata collection solely rely on /proc access
restrictions. Therefore, there is no need to force the send-flags mask on
newly created buses.