Clean-up error constants.

Author: peombwa

src/Authentication/Authentication/ErrorConstants.cs

@@ -19,7 +19,6 @@ internal static class Message
         {
             internal const string InvalidJWT = "Invalid JWT access token.";
             internal const string MissingAuthContext = "Authentication needed, call Connect-Graph.";
-            internal const string InstanceExists = "An instance of {0} already exists. Call {1} to overwrite it.";
             internal const string NullOrEmptyParameter = "Parameter '{0}' cannot be null or empty.";
             internal const string MacKeyChainFailed = "{0} failed with result code {1}.";
         }