libstdc++: [_Hashtable] Optimize destructor

Hashtable destructor do not need to call clear() method that in addition to
destroying all nodes also reset all buckets to nullptr.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (~_Hashtable()): Replace clear call with
	a _M_deallocate_nodes call.

libstdc++-v3/include/bits/hashtable.h

@@ -1664,7 +1664,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		    "Cache the hash code or qualify your functors involved"
 		    " in hash code and bucket index computation with noexcept");
 
-      clear();
+      this->_M_deallocate_nodes(_M_begin());
       _M_deallocate_buckets();
     }