op-mode: T3700: extend completion helper to not list VLAN subinterfaces

This completes commit 0f25f97c9 (op-mode: T3700: add "show evpn access-vlan"
commands) with the latest addition in vyos-utils [1] package to only list
parent interfaces when offering completion help.

[1]: https://github.com/vyos/vyos-utils/commit/675ea7481aeef90