Don't eta expand cons when deriving Data

This eta expansion was introduced with the initial commit for Linear
types.

I believe this isn't needed any longer. My guess is it is an artifact
from the initial linear types implementation: data constructors are
linear, but they shouldn't need to be eta expanded to be used as higher
order functions. I suppose in the early days this wasn't true.

For instance, this works now:

    data T x = T x
    f = \(x :: forall y. y -> T y) -> x True
    f T -- ok!

T is linear, but can be passed where an unrestricted higher order
function is expected. I recall there being some magic around to make
this work for data constructors...

Since this works, there's no need to eta_expand the data constructors in
the derived Data instances.