libbpf: Remove redundant non-null checks on obj_elf

Obj_elf is already non-null checked at the function entry, so remove
redundant non-null checks on obj_elf.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220421031803.2283974-1-cuigaosheng1@huawei.com