[PM] Remove the underspecified 'getRoot' method from CallGraph. It's
only user was an ancient SCC printing bit of the opt tool which really
should be walking the call graph the same way the CGSCC pass manager
does.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195800 91177308-0d34-0410-b5e6-96231b3b80d8