What's a doorway that doesn't have a door called?

If you were just using the dictionary definitions, it would be called a "doorway".

Doorway
1. the passage or opening into a building, room, etc., commonly closed and opened by a door; portal.
2. a means of access: a doorway to success.

Door
1. a movable, usually solid, barrier for opening and closing an entranceway, cupboard, cabinet, or the like, commonly turning on hinges or sliding in grooves.
2. a doorway: to go through the door.
3. the building, house, etc., to which a door belongs: My friend lives two doors down the street.
4. any means of approach, admittance, or access: the doors to learning.
5. any gateway marking an entrance or exit from one place or state to another: at heaven's door.

Passageway
1. a way for passing into, through, or out of something, as within a building or between buildings; a corridor, hall, alley, catwalk, or the like.
2. a corridor on a ship.

Archway
1. an entrance or passage under an arch.
2. a covering or enclosing arch.

Lintel
1. a horizontal architectural member supporting the weight above an opening, as a window or a door.

soffit
1. the underside of an architectural feature, as a beam, arch, ceiling, vault, or cornice.

Source

Since you're dealing with architectural drawings, you'll probably want to use "Cased Opening" (or "Arched Opening" if the top of the opening should have an arch). Which would be drawn on a blueprint using this symbol.

A cased opening is a doorway that is trimmed out, but does not contain a door. If you just want an opening in the wall without trim, you could just call it an "Opening", or "archway". It would look like this on a blueprint.