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