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/ui/doom/config.el

@@ -104,3 +104,6 @@
     nil nil '(center repeated))
   (define-fringe-bitmap 'git-gutter-fr:deleted [128 192 224 240]
     nil nil 'bottom))
+
+;; standardize default fringe width
+(if (fboundp 'fringe-mode) (fringe-mode '4))