samples/bpf: fix trace_output example

samples/bpf: fix trace_output example

llvm cannot always recognize memset as builtin function and optimize
it away, so just delete it. It was a leftover from testing
of bpf_perf_event_output() with large data structures.

Fixes: 313a164274a9 ("samples: bpf: add bpf_perf_event_output example")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>