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new (yet unpublished) tests lean heavily toward integration testing more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

@@ -6,7 +6,7 @@
 ;;      while it is lexically bound, you get "Defining as dynamic an already
 ;;      lexical var" errors in Emacs 28+).
 ;;;###autoload (defvar projectile-project-root nil)
-;;;###autoload (defvar projectile-enable-caching doom-interactive-p)
+;;;###autoload (defvar projectile-enable-caching (not noninteractive))
 ;;;###autoload (defvar projectile-require-project-root 'prompt)
 
 ;;;###autodef