LibGUI: Cast unused smart-pointer return values to void

Author: https://github.com/AtkinsSJ
Commit: d95e50643e
Pull-request: https://github.com/SerenityOS/serenity/pull/11151

Userland/Libraries/LibGUI/UndoStack.cpp

@@ -57,7 +57,7 @@ void UndoStack::push(NonnullOwnPtr<Command> command)
 {
     // If the stack cursor is behind the top of the stack, nuke everything from here to the top.
     while (m_stack.size() != m_stack_index)
-        m_stack.take_last();
+        (void)m_stack.take_last();
 
     if (m_clean_index.has_value() && m_clean_index.value() > m_stack.size())
         m_clean_index = {};