Fix another -Wunused-local-typedef warning in include/__tree

[dim]

Summary

Fix another -Wunused-local-typedef warning in include/__tree. The _Pp
typedef in tree<_Tp, _Compare, _Allocator>::count_multi() isn't used
anywhere, so adding _LIBCPP_UNUSED is unecessary.

libcxx/trunk/include/__tree

__tree<_Tp, _Compare, _Allocator>::__count_unique(const _Key& __k) const
     return 0;
 }
 
 template <class _Tp, class _Compare, class _Allocator>
 template <class _Key>
 typename __tree<_Tp, _Compare, _Allocator>::size_type
 __tree<_Tp, _Compare, _Allocator>::__count_multi(const _Key& __k) const
 {
-    typedef pair<const_iterator, const_iterator> _Pp;
     __node_const_pointer __result = __end_node();
     __node_const_pointer __rt = __root();
     while (__rt != nullptr)
     {
         if (value_comp()(__k, __rt->__value_))
         {
             __result = __rt;
             __rt = static_cast<__node_const_pointer>(__rt->__left_);