Delete SPDs when an IPSec tunnel is deleted. Fix #2719.

- Add new function to delete SPDs (see 'remove_tunnel_spd_policy($phase1,$phase2)' on vpn.inc)
- Change vpn_ipsec.php to delete SPDs on phase 2 and phase 1.
- Change the method GET to delete phase 2 (needs to inform which is the phase 1)

It should fix #2719.