Revert "gdk: enable rgba visuals by default"

This reverts commit a2ccb6601e.

This turns out to break GtkStatusIcon, and invalidates
the documentation of gdk_screen_get_system_visual().

docs/reference/gtk/running.sgml

@@ -478,15 +478,6 @@ nevertheless.
   </para>
 </formalpara>
 
-<formalpara>
-  <title><envar>GDK_RGBA</envar></title>
-
-  <para>
-    Since 3.12, this environment variable can be used to prevent
-    using RGBA visuals on the X11 backend by setting GDK_RGBA=0.
-  </para>
-</formalpara>
-
 <formalpara>
   <title><envar>XDG_DATA_HOME</envar>, <envar>XDG_DATA_DIRS</envar></title>
 

gdk/x11/gdkvisual-x11.c

@@ -285,14 +285,6 @@ _gdk_x11_screen_init_visuals (GdkScreen *screen)
         }
     }
 
-  /* prefer the RGBA visual unless GDK_RGBA=0 */
-  if (x11_screen->rgba_visual)
-    {
-      const gchar *csd_env = g_getenv ("GDK_RGBA");
-      if (g_strcmp0 (csd_env, "0") != 0)
-          x11_screen->system_visual = x11_screen->rgba_visual;
-    }
-
 #ifdef G_ENABLE_DEBUG
   if (_gdk_debug_flags & GDK_DEBUG_MISC)
     {