ggml-alloc : remove buffer_id from leaf_alloc (ggml/987)

This commit removes the buffer_id field from the leaf_alloc struct.

The motivation for is that this field is only written to and never
read/used as far as I can tell. Each tensor_alloc has a buffer_id field
and this is what caused me to look into this more closely, to
understand what the buffer_id in leaf_alloc was used for.