Remove "virtual" and add "override".

Seems clang-modernize couldn't add "override" to nested classes, so
doing it by hand. Also removed unused virtual member function that
is not overriding anything, that seems to have been added by mistake.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205552 91177308-0d34-0410-b5e6-96231b3b80d8