Revert "Bluetooth: hci_sync: add lock to protect HCI_UNREGISTER"

This reverts commit 17aac120.

It breaks the Android KABI and will be brought back at a later time when
it is safe to do so.

Bug: 161946584
Change-Id: I710deec3131b9df951fba75469f7b16653083f78
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>