Remove ordinals from Input Graph elements.

An ordinal is set to each child of Input Graph, but no one actually
uses it. The only piece of code that gets ordinaly values is
sortInputElements in InputGraph.cpp, but it does not actually do
anything -- we assign ordinals in increasing order just before
calling sort, so when sort is called it's already sorted. It's no-op.
We can simply remove it. No functionality change.

Differential Revision: http://llvm-reviews.chandlerc.com/D3270

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205501 91177308-0d34-0410-b5e6-96231b3b80d8