Remove an obsolete comment

Remove a comment that was about a macro that was removed in a7ef1cd35e9347a285f245041db4eb94047f4ab0
auto merge of #2021 : SimonSapin/servo/patch-1, r=jdm

Remove a comment that was about a macro that was removed in a7ef1cd35e9347a285f245041db4eb94047f4ab0