gexp: Keep only a single 'references' field.

The distinction between native inputs and "normal" inputs can already be
determined by looking at the 'native?' field of <gexp-input>.  The extra
'natives' field of <gexp> added complexity for no good reason.

* guix/gexp.scm (<gexp>)[natives]: Remove.
(write-gexp): Remove use of 'gexp-native-references'.
(gexp-inputs)[native-input?]: New procedure.
Use it.
(gexp->sexp)[reference->sexp]: Honor N? for input lists.
Remove use of 'gexp-native-references'.
(gexp)[collect-native-escapes]: Remove.
Simplify.