session-worker: rename response_text to response

Right now we always give text responses back to pam modules,
so the response variable is named response_text.  That's going
to change in the future, when we introduce private protocol for
the module to talk to GDM.

As prep work for that change, rename "response_text" to "response".

https://bugzilla.gnome.org/show_bug.cgi?id=788851