[ELF] Drop unused original symbol after wrapping if not defined

We were previously only omitting the original of a wrapped symbol if it
was not used by an object file and undefined. We can tighten the second
condition to drop any symbol that isn't defined instead, which lets us
drop a previous check (added in https://reviews.llvm.org/D118756) that
was only covering some such symbols.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D124065