samples: net: zperf: Remove deadcode

As we are checking that pkt is not NULL already in the start
of the function, remove the checks later in the code in function
zperf_tcp_receiver.c:tcp_received()

Coverity-CID: 179248
Fixes #5061

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>