Bluetooth: Remove unneeded hci_cc_read_ssp_mode function

The kernel has no need to track the hci_read_ssp_mode command since it
has the hci_sent_cmd_data function to check what value was set when
hci_write_ssp_mode completes.

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