Remove unused function from print-changed=dot-cfg code

Summary:
Remove unused function from print-changed=dot-cfg code to silence a
gcc compiler warning.

Author: Jamie Schmeiser <[email protected]>
Reviewed By: uabelho(Mikael Holmen)
Differential Revision: https://reviews.llvm.org/D113188

Author: jamieschmeiser

llvm/lib/Passes/StandardInstrumentations.cpp

@@ -1553,8 +1553,6 @@ class DotCfgDiff {
 protected:
   // Return the string surrounded by HTML to make it the appropriate colour.
   std::string colourize(std::string S, IRChangeDiffType T) const;
-  // Return the string containing the colour to use as a Dot attribute.
-  std::string attribute(IRChangeDiffType T) const;
 
   void createNode(StringRef Label, const BlockDataT<DCData> &BD,
                   IRChangeDiffType T) {
@@ -1646,10 +1644,6 @@ std::string DotCfgDiff::colourize(std::string S, IRChangeDiffType T) const {
   return "<FONT COLOR=\"" + Colours[T] + "\">" + S + "</FONT>";
 }
 
-std::string DotCfgDiff::attribute(IRChangeDiffType T) const {
-  return "color=" + Colours[T];
-}
-
 std::string DotCfgDiffDisplayGraph::attribute(IRChangeDiffType T) const {
   return "color=" + Colours[T];
 }