GDK W32: Remove an unnecessary type check

No idea why it's here, the hash table can store any kind of data,
there's no reason why it wouldn't be able to store an old X string type.
Might be a holdout from the old days, when strings were handled in
a special way (stored directly in the clipboard?).

https://bugzilla.gnome.org/show_bug.cgi?id=786509

gdk/win32/gdkselection-win32.c

@@ -757,8 +757,6 @@ selection_property_store (GdkWindow *owner,
   GdkSelProp *prop;
   GdkWin32Selection *win32_sel = _gdk_win32_selection_get ();
 
-  g_return_if_fail (type != GDK_TARGET_STRING);
-
   prop = g_hash_table_lookup (win32_sel->sel_prop_table, GDK_WINDOW_HWND (owner));
 
   if (prop != NULL)