[ELF] Delete a redundant assignment to SectionBase::assigned. NFC
[ELF] Delete a redundant assignment to SectionBase::assigned. NFC

LinkerScript::discard marks a section dead. It is unnecessary to set the
`assigned` bit.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@371804 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld