Remove ut_align() and use aligned_malloc()

Before the aligned_malloc() portability wrapper was introduced, InnoDB used a pattern of over-allocating a memory block and then invoking ut_align() to guarantee the desired alignment.

It is cleaner to invoke aligned_malloc() and aligned_free() directly. Because there is no legitimate use of ut_align(), it should be removed. In assertions, ut_align_down() can be used instead.