box: zap field_map_get_size

Turns out we don't really need it as we can use data_offset + bsize
(i.e. the value returned by tuple_size() helper function) to get the
size of a tuple to free. We only need to take into account the offset
of the base tuple struct in the derived struct (memtx_tuple).

There's a catch though:

 - We use sizeof(struct memtx_tuple) + field_map_size + bsize for
   allocation size.
 - We set data_offset to sizeof(struct tuple) + field_map_size.
 - struct tuple is packed, which makes its size 10 bytes.
 - memtx_tuple embeds struct tuple (base) at 4 byte offset, but since
   it is not packed, its size is 16 bytes, NOT 4 + 10 = 14 bytes as
   one might expect!
 - This means data_offset + bsize + offsetof(struct memtx_tuple, base)
   doesn't equal allocation size.

To fix that, let's mark memtx_tuple packed. The only side effect it has
is that we save 2 bytes per each memtx tuple. It won't affect tuple data
layout at all, because struct memtx_tuple already has a packed layout
and so 'packed' will only affect its size, which is only used for
computing allocation size.

My bad I overlooked it during review.

Follow-up f1d9f257 ("box: introduce multikey indexes in memtx").