net: zperf: unify get_session among TCP and UDP

The modules zperf_udp_receiver and zperf_tcp_receiver use two
different functions to get a zperf session to store the
statistics, there is a TODO comment in the zperf_session module
suggesting to unify that part.
So delete the get_tcp_session function and use get_session for
both TCP and UDP receiver module.
Delete sock field from struct session because it's not used
anymore.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>