Bugfix: Don’t use ->old_parent for floating cons (Thanks eelvex)

Instead, we attach them to their workspace when toggling back to tiling. This
makes more sense; afterall, floating clients are always directly below a
CT_WORKSPACE container.

include/data.h

@@ -275,8 +275,6 @@ struct Con {
     enum { CT_ROOT = 0, CT_OUTPUT = 1, CT_CON = 2, CT_FLOATING_CON = 3, CT_WORKSPACE = 4 } type;
     orientation_t orientation;
     struct Con *parent;
-    /* parent before setting it to floating */
-    struct Con *old_parent;
 
     struct Rect rect;
     struct Rect window_rect;