[ELF] - ICF: Remove dead code. NFC.

Code is dead. We use only InputSections when building the list of
sections elegible for the ICF:
https://github.com/llvm-mirror/lld/blob/master/ELF/ICF.cpp#L439
And 'isEligible' filters out SyntheticSections as well for us. 

That way the only Kind we have in the Sections vector is SectionBase::Regular,
so we do not need to check sections kind at all, it is always the same.

llvm-svn: 335460