[ELF] - Eliminate dead code from shouldKeepInSymtab. NFC.

shouldKeepInSymtab is called from copyLocalSymbols:
https://github.com/llvm-mirror/lld/blob/master/ELF/Writer.cpp#L574

The pre-condition is that symbol should be Defined:
https://github.com/llvm-mirror/lld/blob/master/ELF/Writer.cpp#L572

And its section is Live:
https://github.com/llvm-mirror/lld/blob/master/ELF/Writer.cpp#L548

InputSection::Discarded section can never be Live. And hence I believe
check I removed in this patch is excessive.

llvm-svn: 335973