It is/was a temporary brace, it is safe to remove it.
During the fabrication of the wall, or before the floor system was added overhead the brace was added to stabilize that small wall section, perhaps before the beam was added. The beam in the picture, typically goes from foundation wall to foundation wall, although there can be many exceptions. Still, this hold true... the floor system is now in, that in itself is a huge brace with al the plywood set to the joists that are tied to the beam. Even if the beam that is over the wall in question was not tied into either wall, the floor alone would not allow the beam to move, and therefore the wall in question to rack.
If there is a concern for this non-bearing wall to move or shift or whatever.... and it is a non bearing wall because of the beam over it and a steel post beside the wall to support the beam..., albeit short, the diagonal of the stair carriage WILL hold the center of the studs sable if the carpenter who set the stairs did his job and fastened them to the framing the way he should. The landing does the rest.
As a mention, the concrete floor has a leveling layer of concrete over it. Not only the end of the diagonal in in the floor, the plate it is nailed to is in the floor as well, by about 3/4".