selftest/bpf: Fix compilation warning in sockmap_parse_prog.c

Remove unused len variable, which causes compilation warnings.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200314001834.3727680-1-andriin@fb.com