Alignment of 8 is not required for additional header

malloc guarantees aligned memory. If we add an alignment here, we are adding
additonal unused 4 bytes. Each allocator has its own 4/8 byte header
(GGC / ARM have 4 bytes).

So if user request for 8 bytes of memory stats will add 8 + allocator 8.
However if we remove the alignment in stats header, allocator will consider
add 4 bytes to 12 byte request and zero padding.

It will be beneficial to leave the padding to allocator.

platform/mbed_alloc_wrappers.cpp

@@ -40,10 +40,8 @@ are active, the second one (MBED_MEM_TRACING_ENABLED) will trace the first one's
 /* Implementation of the runtime max heap usage checker                       */
 /******************************************************************************/
 
-/* Size must be a multiple of 8 to keep alignment */
 typedef struct {
     uint32_t size;
-    uint32_t pad;
 } alloc_info_t;
 
 #ifdef MBED_HEAP_STATS_ENABLED