I've removed those gratuitous context uses in Objects/longobject.c
in r72202 (trunk) and r72203 (py3k).

I'm not sure it's a good idea to remove the context field completely.  
Apart from the compatibility issues that Antoine pointed out, it
doesn't seem improbable that some getters and setters might need it.

A Google code search for PyGetSetDef turned up at least one non-null
use of the context field (in something called pyephem).  I didn't
look at it closely, though.