cmSetCommand: Re-use local named variable.

Source/cmSetCommand.cxx

@@ -59,7 +59,7 @@ bool cmSetCommand
   // SET (VAR) // Removes the definition of VAR.
   if (args.size() == 1)
     {
-    this->Makefile->RemoveDefinition(args[0]);
+    this->Makefile->RemoveDefinition(variable);
     return true;
     }
   // SET (VAR PARENT_SCOPE) // Removes the definition of VAR