InnoDB: Remove HAVE_purify, UNIV_INIT_MEM_TO_ZERO, UNIV_SET_MEM_TO_ZERO.

The compile-time setting HAVE_purify can mask potential bugs.
It is being set in PB2 Valgrind runs. We should simply get rid of it,
and replace it with UNIV_MEM_INVALID() to declare uninitialized memory
as such in Valgrind-instrumented binaries.

os_mem_alloc_large(), ut_malloc_low(): Remove the parameter set_to_zero.

ut_malloc(): Define as a macro that invokes ut_malloc_low().

buf_pool_init(): Never initialize the buffer pool frames. All pages
must be initialized before flushing them to disk.

mem_heap_alloc(): Never initialize the allocated memory block.

os_mem_alloc_nocache(), ut_test_malloc(): Unused function, remove.

rb:813 approved by Jimmy Yang