Bluetooth: Remove ssp_debug_mode debugfs option

The ssp_debug_mode debugfs option for developers is no longer
needed. Support for using Secure Simple Pairing (SSP) debug
mode is exposed by the management interface now.

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