user: avoid database work on superfluous pin

The only thing this does is changing the updated_at field of the user.
Afaict this came to be because prior to pins federating this was split
into two functions, one of which created a changeset, the other applying
a given changeset. When this was merged the bits were just copied into
place.

lib/pleroma/user.ex

@@ -2581,10 +2581,10 @@ def add_pinned_object_id(%User{} = user, object_id) do
           [pinned_objects: "You have already pinned the maximum number of statuses"]
         end
       end)
+      |> update_and_set_cache()
     else
-      change(user)
+      {:ok, user}
     end
-    |> update_and_set_cache()
   end
 
   @spec remove_pinned_object_id(User.t(), String.t()) :: {:ok, t()} | {:error, term()}