[ELF] Replace a dead test in getSymVA() with assert()

Symbols relative to discarded comdat sections are Undefined instead of
Defined now (after D59649 and D61583). The `== &InputSection::Discarded`
test becomes dead. I cannot find a test related to this behavior.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62725

llvm-svn: 362218