loginDialog: don't ever call _reset directly

The only time we ever call _reset directly is when
detecting changes to disable-user-list.  We can implicitly
trigger a reset for this case, just as easily by calling
this._authPrompt.reset()

This commit makes that change for consistency and to make
it easier to adjust the authprompt workflow later.

https://bugzilla.gnome.org/show_bug.cgi?id=683437