printf-hook: Avoid double-free when freeing Vstr config

Thread-specific objects get freed when the thread value object is
destroyed (wasn't the case earlier, i.e. before 2b19dd35), which
may cause the second call to vstr_free_conf() to fail in an assert
in Vstr (depending on how it was built).

src/libstrongswan/utils/printf_hook.c

@@ -474,7 +474,6 @@ METHOD(printf_hook_t, destroy, void,
 	/* freeing the Vstr_conf of the main thread */
 	vstr_conf->destroy(vstr_conf);
 	vstr_conf = NULL;
-	vstr_free_conf(conf);
 	vstr_exit();
 #endif
 	free(this);