[fixed]XLN symbol on Arcanis, the Omnipotent vanguard

[Korath]

Describe the bug:
Arcanis, the Omnipotent vanguard has a spurious Ixalan common expansion symbol in the upper right of its rules box.

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
Today's dev build, ae31685.

What exactly should be the correct behavior/interaction?
No symbol. Obviously.

Are any other cards possibly affected by this bug?
None of the other vanguards have it, the card is properly filtered out if I filter for XLN-only, and the card's not flagged as being in XLN (or any expansion - none of the vanguard cards are).

If I do put it into a (different) expansion, the appropriate expansion symbol appears in the same place. So I guess this could be hacked around by positioning the expansion symbol on vanguards somewhere invisible. But that won't tell me why the symbol shows up on this card and none of the others, and why that particular symbol.

Don't know if this is deckbuilder-only; the card's not enabled in either Manalink or Shandalar, so that's the only place I can conveniently see it.

[Korath]

Caused by the bit of hackery that lets fake cards like Blinkmoth and Purphoros, God of the Forge Incarnate have expansion symbols, despite not being part of an expansion themselves - the id before Arcanis's is Wind Strider. I've special-cased it out in f900ff4, the same way Face-Down Creature, Spawn of Azar, the Archenemy format card, and Handicap 3 Cards are.