WB_FORCE_SELECTION is dead

at least as far back as:

    commit ac7acb0a
    "Merged SvTreeListBox and SvLBox."

Change-Id: I13d8ab413365928749b1c59a0cca0db115b80ffa
Reviewed-on: https://gerrit.libreoffice.org/41280Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

include/svtools/treelistbox.hxx

@@ -158,8 +158,6 @@ public:
     virtual void        Clone(SvLBoxItem* pSource) = 0;
 };
 
-#define WB_FORCE_SELECTION          ((WinBits)0x8000)
-
 enum class DragDropMode
 {
     NONE            = 0x0000,