tools/bpftool: Fix skeleton codegen

Remove unnecessary check at the end of codegen() routine which makes codegen()
to always fail and exit bpftool with error code. Positive value of variable
n is not an indicator of a failure.

Fixes: 2c4779ef ("tools, bpftool: Exit on error in function codegen")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/bpf/20200612201603.680852-1-andriin@fb.com