net: tcp: Avoid unnecessary packet cloning on the RX path

So far, TCP cloned a packet with data on an RX path for the application,
leaving the original packet intact. This isn't really needed, as the
original packet is unconditionally freed later anyway, so the TCP can as
well simply queue the original packet for the application, while
informing the network processing core, that the packet was consumed by
the TCP layer.

This allows to improve the download throughput even further, since the
CPU don't waste time on needles packet copying.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>