Remove doom-fringe-size; set default fringe in :ui doom

There was no way to customize doom-fringe-size. Better to customize
fringes by using fringe-mode, {left,right}-fringe-width or
set-window-fringes directly.

modules/feature/popup/autoload.el

@@ -277,7 +277,7 @@ that window has been changed or closed."
 (defun +popup|adjust-fringes ()
   "Hides the fringe in popup windows, restoring them if `+popup-buffer-mode' is
 disabled."
-  (let ((f (if +popup-buffer-mode 0 doom-fringe-size)))
+  (let ((f (if +popup-buffer-mode 0)))
     (set-window-fringes nil f f fringes-outside-margins)))
 
 ;;;###autoload