Revert "xl2tpd: rename netifd protocol handler to l2tpv2 to avoid clashes with l2tpv3"

This reverts commit 981f2da45747df72c5ae2c45f265fe8f29af3d52.
OpenWrt wiki knew l2tp from l2tpv3tun, but luci knew l2tp from xl2tpd