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new (yet unpublished) tests lean heavily toward integration testing more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.

bin/doom

@@ -230,7 +230,7 @@ SEE ALSO:
   (dolist (file loads)
     (load (doom-path (cdr file))
           (not (equal (car file) "--strict-load"))
-          (not doom-debug-p) t))
+          (not init-file-debug) t))
   (dolist (form evals)
     (eval (read (cdr form)) t)))