vyatta-cfg-vpn: remove the cfgvti helper program

The cfgvti helper program was originally added for configuring VTIs.
The functionality it provided is now included upstream in iproute, so
it is no longer required following the previous commits for Bug #358.

Bug #358 http://bugzilla.vyos.net/show_bug.cgi?id=358
0.12.105+vyos2+lithium4