Bluetooth: Remove support for other SMP keys than the LTK

For now, only the LTK is properly supported. We are able to receive
and generate the other types of keys, but we are not able to use
them. So it's better not request them to be distributed.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>