viewport: Remove unused frame

We've always removed this with Adwaita.

gtk/gtkviewport.c

@@ -475,7 +475,6 @@ gtk_viewport_init (GtkViewport *viewport)
   viewport->hadjustment = NULL;
   viewport->vadjustment = NULL;
 
-  gtk_widget_add_css_class (widget, "frame");
   viewport_set_adjustment (viewport, GTK_ORIENTATION_HORIZONTAL, NULL);
   viewport_set_adjustment (viewport, GTK_ORIENTATION_VERTICAL, NULL);
 }

gtk/theme/Adwaita/_common.scss

@@ -3020,10 +3020,6 @@ actionbar > revealer >  box {
 }
 
 scrolledwindow {
-  > viewport.frame { // avoid double borders when viewport inside scrolled window
-    border-style: none;
-  }
-
   // This is used when content is touch-dragged past boundaries.
   // draws a box on top of the content, the size changes programmatically.
   > overshoot {