[ELF] Delete SectionBase::assigned

D67504 removed uses of `assigned` from OutputSection::addSection, which
makes `assigned` purely used in processSectionCommands() and its
callees. By replacing its references with `parent`, we can remove
`assigned`.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D67531

llvm-svn: 372735