modpost: remove ElF_Rela variables from for-loop in section_rel(a)

Remove the Elf_Rela variables used in the for-loop in section_rel().

This makes the code consistent; section_rel() only uses Elf_Rel,
section_rela() only uses Elf_Rela.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>