libbpf: Cleanup linker_append_elf_relos

Clang Static Analyser (scan-build) reports some unused symbols and dead
assignments in the linker_append_elf_relos function. Clean these up.

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/c5c8fe9f411b69afada8399d23bb048ef2a70535.1677658777.git.vmalik@redhat.com