LibGUI: Run TextEditor::on_change callback immediately

This is the only Widget that ran its callback in deferred_invoke(). It
seems to be a holdover from when syntax-highlighting ran whenever the
text changed, but that has not been true since
bec2b3086c. Running the callback
immediately has no obvious downsides, but does make it a lot easier to
reason about. (I might have spent an hour confused as to why things
were happening in the wrong order...)

Userland/Libraries/LibGUI/TextEditor.h

@@ -361,7 +361,6 @@ private:
     bool m_ruler_visible { false };
     bool m_gutter_visible { false };
     bool m_needs_rehighlight { false };
-    bool m_has_pending_change_notification { false };
     bool m_automatic_indentation_enabled { false };
     WrappingMode m_wrapping_mode { WrappingMode::NoWrap };
     bool m_visualize_trailing_whitespace { true };