Bluetooth: Delete unused hci_req_prepare_suspend() declaration

hci_req_prepare_suspend() has been deprecated in favor of
hci_suspend_sync().

Fixes: 182ee45d

 ("Bluetooth: hci_sync: Rework hci_suspend_notifier")
Signed-off-by: Yao Xiao <xiaoyao@rock-chips.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>