Remove "this->".

For most member function calls we do not use "this->" in this file.
Remove the rest for consistency.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205550 91177308-0d34-0410-b5e6-96231b3b80d8
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