Clean up call of writemap() in ac_encounter()

Clean up call of writemap() in ac_encounter()

Pass the more obviously correct plane_owner instead of player->cnum.
They're the same, actually.

When deities could still fly foreign planes (before commit 2023b47c),
they weren't.  ac_encounter() updated the plane owner's in-memory
bmap, but wrote the current player's bmap to disk.