[NTG-context] simplify \setuppapersize call

[Sanjoy Mahajan]

> maybe, but i'm hesitant to change this because it has been so from the 
> beginning

Reasonable enough.

Though the mixed A4/letter dimensions with \setuppapersize[letter] is
probably not what a user intends and not useful for backward
compatibility.  They should probably get a PDF with an a4 whateverbox
(mediabox?).

-Sanjoy

`Never underestimate the evil of which men of power are capable.'
         --Bertrand Russell, _War Crimes in Vietnam_, chapter 1.