Bluetooth: Host: add unregister connection callback function

[Description]
tests: shell: Restart bt will register the same connection callback twice.
Callback next node point to itself, when link established callback function
loop infinitely.
[Fix]
Unregister the previous callback to avoid register repeatedly.
[Test]
After bt init/disable times, create connection successfully.

Signed-off-by: huajiang zheng <huajiang.zheng@nxp.com>