Remove or fix ut_timer_now()

The function ut_init_timer() is never called in MariaDB 10.2.2 or later, ever since the merge of InnoDB from MySQL 5.7.9. Because of this, the function pointer ut_timer_now() should point to ut_timer_none(), which will always return 0. The only caller appears to be btr_defragment_thread(). One of the effects could be that btr_defragment_item_t::last_processed is always a constant 0.

I would prefer to remove the compilation unit and to fix the defragmentation in some way so that it does not depend on the timestamp. We should perhaps test the performance of defragmentation compared to MariaDB 10.1, where the feature was introduced by MDEV-5834.