kldxref: Don't warn and skip file if no relocations are found

This case is hit for the kernel itself on riscv64, and did not used to
be checked. Since the code here can already handle missing Elf_Rel
and/or Elf_Rela just delete the check.

Reviewed by: jhb, imp
Fixes: 0299afdff145 ("kldxref: Make use of libelf to be a portable cross tool")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46517