Bluetooth: Host: Remove HCI ECC emulation

Remove the HCI command & event emulation layer for ECDH commands and
events. This means that we always do the necessary operations in the host.
The existing BT_ECC Kconfig option stays, but now gets automatically
enabled when necessary (e.g. based on the BT_SMP option), which is why this
commit removes so many explicit assignments in prj.conf files.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>