Bluetooth: Remove unnecessary SMP STK define

We never store the "master" type of STKs since we request encryption
directly with them so we only need one STK type (the one that's
looked-up on the slave side). Simply remove the unnecessary define and
rename the _SLAVE one to the shorter form.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>