More WinLinkInputGraph cleanup.

This function is called only from WinLinkDriver.cpp.
Move the function to that file and mark as static.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@224211 91177308-0d34-0410-b5e6-96231b3b80d8
Remove PECOFFLibraryNode.

This class is empty, provides no additional feature to the base class.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@224212 91177308-0d34-0410-b5e6-96231b3b80d8