modpost: drop executable ELF support

Since commit 269a535c ("modpost: generate vmlinux.symvers and
reuse it for the second modpost"), modpost only parses relocatable
files (ET_REL).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>