Go back to not using custom-push-theme when enabling a theme

Go back to not using custom-push-theme when enabling a theme

* lisp/custom.el (enable-theme): Relying on custom-push-theme to
handle theme settings and prior user settings was a mistake.  The
theme settings haven't changed between loading the theme and enabling
it, so we don't need all of what custom-push-theme does.  However, we
still need to save a user setting outside of Customize, in order to be
able to get back to it, so do that in enable-theme itself.