I think this was used to secure a weapon in the property I bought. It is like a flat drive, but only clockwise. One of four fixing was with a normal screw, which I removed.
These are security screws and are designed to not be removed.
Common methods are:
- Drill the screw head out
- Cut a new slot
- Use extractor tools (these are of questionable utility)
- Grasp the sides of the head (hard, it was designed to resist this kind of removal)
- Get behind the screw and cut it off. This is a good option if you're going to tear out the wood on the wall anyway.
In your case, could you consider leaving it in place? Even if you do not have the small arms it was originally designed to secure, having a solid, secure mounting point can secure things like valuables, bikes, strongboxes, or safes.