VideoCommon/BoundingBox: Move PixelShaderManager::SetBoundingBoxActive() calls into Enable()/Disable()

Now that we have an actual interface to manage things, we can stop
duplicating the calls to to the pixel shader manager and remove the
need to remember to actually do so when disabling or enabling the
bounding box.

Source/Core/VideoCommon/BoundingBox.cpp

@@ -9,6 +9,7 @@
 
 #include "Common/ChunkFile.h"
 #include "Common/CommonTypes.h"
+#include "VideoCommon/PixelShaderManager.h"
 
 namespace BoundingBox
 {
@@ -29,11 +30,13 @@ std::array<u16, 4> s_coordinates{
 void Enable()
 {
   s_is_active = true;
+  PixelShaderManager::SetBoundingBoxActive(s_is_active);
 }
 
 void Disable()
 {
   s_is_active = false;
+  PixelShaderManager::SetBoundingBoxActive(s_is_active);
 }
 
 bool IsEnabled()