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new (yet unpublished) tests lean heavily toward integration testing more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

modules/editor/snippets/config.el

@@ -34,7 +34,7 @@
 
   ;; Reduce verbosity. 3 is too chatty about initializing yasnippet. 2 is just
   ;; right (only shows errors).
-  (setq yas-verbosity (if doom-debug-p 3 0))
+  (setq yas-verbosity (if init-file-debug 3 0))
 
   ;; default snippets library, if available
   (add-to-list 'load-path +snippets-dir)