[ELF] getRelocatedSection: remove the check for ET_REL object file

getRelocatedSection interface should not check that the object file is
relocatable, as executable files may have relocations preserved with
`--emit-relocs` linker flag. The relocations are useful in context of post-link
binary analysis for function reference identification. For example, BOLT relies
on relocations to perform function reordering.

Reviewed By: MaskRay, jhenderson

Differential Revision: https://reviews.llvm.org/D102296