gdk: Remove GDK_SURFACE_DRAG enum member

I want to remove the whole enum, but this value was used all over the
place. So removing it first ensures I didn't forget anything.

gdk/broadway/gdksurface-broadway.c

@@ -247,7 +247,6 @@ gdk_broadway_display_create_surface (GdkDisplay     *display,
                               "display", display,
                               NULL);
       break;
-    case GDK_SURFACE_DRAG:
     default:
       g_assert_not_reached ();
       break;

gdk/gdksurfaceprivate.h

@@ -30,7 +30,6 @@ typedef enum
 {
   GDK_SURFACE_TOPLEVEL,
   GDK_SURFACE_POPUP,
-  GDK_SURFACE_DRAG
 } GdkSurfaceType;
 
 struct _GdkSurface

gdk/macos/gdkmacossurface.c

@@ -608,7 +608,6 @@ _gdk_macos_surface_new (GdkMacosDisplay *display,
 
       break;
 
-    case GDK_SURFACE_DRAG:
     default:
       g_warn_if_reached ();
       ret = NULL;

gdk/wayland/gdksurface-wayland.c

@@ -878,12 +878,6 @@ gdk_wayland_display_create_surface (GdkDisplay     *display,
                               "display", display,
                               NULL);
       break;
-    case GDK_SURFACE_DRAG:
-      g_warn_if_fail (parent == NULL);
-      surface = g_object_new (GDK_TYPE_WAYLAND_DRAG_SURFACE,
-                              "display", display,
-                              NULL);
-      break;
     default:
       g_assert_not_reached ();
       break;

gdk/win32/gdksurface-win32.c

@@ -455,7 +455,6 @@ gdk_win32_display_create_surface (GdkDisplay     *display,
                               "display", display,
                               NULL);
       break;
-    case GDK_SURFACE_DRAG:
     default:
       g_assert_not_reached ();
       break;

gdk/x11/gdksurface-x11.c

@@ -1259,7 +1259,6 @@ gdk_x11_display_create_surface (GdkDisplay     *display,
                               "display", display,
                               NULL);
       break;
-    case GDK_SURFACE_DRAG:
     default:
       g_assert_not_reached ();
       break;