UI: Remove unused ExpandCheckBox

ExpandCheckBox was introduced in 88b6c63, but the seemingly replaced by
SourceTreeSubItemCheckBox during development. This means that it became
completely unused.

UI/cmake/legacy.cmake

@@ -172,7 +172,6 @@ target_sources(
           item-widget-helpers.hpp
           context-bar-controls.cpp
           context-bar-controls.hpp
-          expand-checkbox.hpp
           focus-list.cpp
           focus-list.hpp
           hotkey-edit.cpp

UI/cmake/ui-elements.cmake

@@ -11,7 +11,6 @@ target_sources(
           context-bar-controls.hpp
           double-slider.cpp
           double-slider.hpp
-          expand-checkbox.hpp
           focus-list.cpp
           focus-list.hpp
           horizontal-scroll-area.cpp

UI/expand-checkbox.hpp

@@ -1,7 +0,0 @@
-#pragma once
-
-#include <QCheckBox>
-
-class ExpandCheckBox : public QCheckBox {
-	Q_OBJECT
-};

UI/source-tree.cpp

@@ -4,7 +4,6 @@
 #include "qt-wrappers.hpp"
 #include "visibility-checkbox.hpp"
 #include "locked-checkbox.hpp"
-#include "expand-checkbox.hpp"
 #include "platform.hpp"
 
 #include <obs-frontend-api.h>