refactor: remove unused core variables

doom-debug-p and doom-interactive-p have always been intentionally
redundant, because changing the variables they replaced had other
side-effects, which made writing tests for them difficult. Since our
new (yet unpublished) tests lean heavily toward integration testing more
than unit testing, this becomes an implementation detail.

And doom-init-p's only use was refactor out at some point in the past,
so it's no longer used.

Also done to reduce Doom's footprint, in general.

core/autoload/scratch.el

@@ -111,7 +111,7 @@ the first, fresh scratch buffer you create. This accepts:
     (remove-hook 'doom-switch-buffer-hook #'doom-persist-scratch-buffers-after-switch-h)))
 
 ;;;###autoload
-(when doom-interactive-p
+(unless noninteractive
   (add-hook 'kill-emacs-hook #'doom-persist-scratch-buffers-h))