selftests/bpf: remove useless bpf_trace_printk

sockmap test is using two programs that use bpf_trace_printk()
which prints into trace_pipe, but nothing is reading it.
Remove it.

Fixes: 6f6d33f3 ("bpf: selftests add sockmap tests")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>