Remove --enable-legacy functionality

As mentioned in #1488, I would like to propose removing the ability to do ``--enable-legacy`` altogether. I've never come across users that make use of it, and as the APIs of Astropy and other packages diverges in future, I think it will actually no longer be correct to enable these legacy layers anyway. Is there any benefit to keeping them around? (as shown in #1488, this functionality broke, and none of us caught it, so if we want to keep it, we have to test it more).