Remove duplicate branch in ColorTranslator.ToHtml (dotnet/corefx#40337)

Commit migrated from dotnet/corefx@87d6aa5

Author: stephentoub

src/libraries/Common/src/System/Drawing/ColorTranslator.cs

@@ -307,8 +307,6 @@ public static string ToHtml(Color c)
                         colorString = "background";
                         break;
                     case KnownColor.Control:
-                        colorString = "buttonface";
-                        break;
                     case KnownColor.ControlLight:
                         colorString = "buttonface";
                         break;