Remove unused meta_stack_get_bottom

Unused since 1b3a58c951 ("...").

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3154>

src/core/stack.c

@@ -942,20 +942,6 @@ meta_stack_get_top (MetaStack *stack)
     return NULL;
 }
 
-MetaWindow *
-meta_stack_get_bottom (MetaStack  *stack)
-{
-  GList *link;
-
-  stack_ensure_sorted (stack);
-
-  link = g_list_last (stack->sorted);
-  if (link != NULL)
-    return link->data;
-  else
-    return NULL;
-}
-
 MetaWindow *
 meta_stack_get_above (MetaStack  *stack,
                       MetaWindow *window,