selftests/bpf: Avoid duplicate btf__parse() call

btf__parse() is repeated after successful setup, leaving the first
instance leaked. Remove redundant and premature call.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20211107165521.9240-8-andrii@kernel.org