[InputGraph] remove unused functions.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194359 91177308-0d34-0410-b5e6-96231b3b80d8
[cleanup] remove readFile and replace with getBuffer.

no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194360 91177308-0d34-0410-b5e6-96231b3b80d8