Bluetooth: MGMT: Remove unused mgmt_pending_find_data

mgmt_pending_find_data() last use was removed in 2021 by
commit 5a750137 ("Bluetooth: hci_sync: Convert MGMT_OP_GET_CLOCK_INFO")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>