gluon-mesh-vpn-tunneldigger: drop package (#3109)

The Gluon Tunneldigger protocol handler hasn't received the care and
attention a core package, especially a VPN protocol, should.

Due to the works in #2102 it's no longer required to maintain
VPN handlers in the Gluon Core and they can be instead included via a
package feed.

Users who wish to continue using a supported core Gluon package are
encouraged to take a look at fastd in conjunction with it's null@l2tp method.
It offers roughly the same performance (it's using the same principle
of operation) and supports connections via IPv6 as a bonus.

In general it should be possible to change the VPN used protocol
asynchronously with a normal update.

If Gluon users wish to continue using tunneldigger the recomendation
is to participate in the development of tunneldigger inside our
community-packages.