Bluetooth: Mesh: get rid of host dependency for dh key for mesh

Commit gets rid of host dependency to generate DH key.
Mesh uses its own function for it that has synchronous
behavior and correct endianism. It simplifies the provisioning
state machine since it doesn't require waiting for the host HCI
handler.
Also, it removes hidden cross-dependency between BLE Mesh and
SMP in the aspect of competition for the same DH key
(https://github.com/zephyrproject-rtos/zephyr/issues/23292

)

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>