selftest/bpf/benchs: Remove an unused header

The benchs/bench_bpf_hashmap_full_update.c doesn't set a custom argp,
so it shouldn't include the <argp.h> header.
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230213091519.1202813-5-aspsk@isovalent.com

tools/testing/selftests/bpf/benchs/bench_bpf_hashmap_full_update.c

 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2022 Bytedance */
 
-#include <argp.h>
 #include "bench.h"
 #include "bpf_hashmap_full_update_bench.skel.h"
 #include "bpf_util.h"