Refactor core-ui

+ Remove doom/switch-theme (replaced with an advice for load-theme)
+ Reorganize core-ui
+ Fix Emacs 26+ hl-line hack being applied for Emacs 25 users
+ Rename doom|show-whitespace-maybe to doom|highlight-non-default-indentation
+ Disable tool-bar, menu-bar and vertical-scroll-bars via
  default-frame-alist; this is a little faster than using the minor
  modes.

core/autoload/buffers.el

@@ -199,18 +199,6 @@ regex PATTERN. Returns the number of killed buffers."
 ;; Hooks
 ;;
 
-;;;###autoload
-(defun doom|protect-visible-buffer ()
-  "Don't kill the current buffer if it is visible in another window (bury it
-instead). Meant for `kill-buffer-query-functions'."
-  (not (and (delq (selected-window) (get-buffer-window-list nil nil t))
-            (not (member (substring (buffer-name) 0 1) '(" " "*"))))))
-
-;;;###autoload
-(defun doom|protect-fallback-buffer ()
-  "Don't kill the scratch buffer. Meant for `kill-buffer-query-functions'."
-  (not (eq (current-buffer) (doom-fallback-buffer))))
-
 ;;;###autoload
 (defun doom|mark-buffer-as-real ()
   "Hook function that marks the current buffer as real."