libbpf: remove unused libbpf_strerror_r and STRERR_BUFSIZE

libbpf_strerror_r() is not exposed as public API and neither is it used
inside libbpf itself. Remove it altogether.

Same for STRERR_BUFSIZE, it's just an orphaned leftover constant which
we missed to clean up some time earlier.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20251001171326.3883055-3-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>