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new (yet unpublished) tests lean heavily toward integration testing more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

@@ -86,8 +86,6 @@ Why this over exec-path-from-shell?
                   "Regenerating"
                 "Generating"))
       (print-group!
-       (when doom-interactive-p
-         (user-error "doom env: must be run on the command line, not an interactive session"))
        (goto-char (point-min))
        (insert
         ";; -*- mode: lisp-interaction; coding: utf-8-unix; -*-\n"