libbpf: Remove redundant check in btf_fixup_datasec()

The check 't->size && t->size != size' is redundant because if t->size
compares unequal to 0, we will just skip straight to sorting variables.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220220072750.209215-1-ytcoode@gmail.com