Is it safe to remove this wall transformer?
1 - Figure out what fuse/breaker controls that set of outlets and turn it off. That is always the official advice before removing an outlet cover, but especially important here as you don't know what you will find.

2 - Remove the outlet cover. If the wires go out of the junction box separately from the regular AC power cable then you should be able to simply cut off the wires from the adapter and push the ends of the wires outside the junction box. That way you will not have any loose wires hanging around 120V waiting to get accidentally energized and zap something on the other end.

3 - If you find that the wires from the adapter are exiting the junction box together with the regular AC power cable then do NOT cut the wires (yet). You will need to find the other end and, ideally, remove the entire cable so that there are no loose wires next to regular AC power cables. Alternatively, you could put wire nuts on the ends of the wire after cutting off the adapter.

Whether or not it is allowed by code (I suspect not), I have seen this type of connection many times. It is never a good idea to run low voltage wiring (telephone, alarm, network, security camera, etc.) parallel to (or in the same conduit as) AC line voltage wiring. Most of the time I have just seen the alarm installers cheat a little and the wire exits the box (or even goes around the box behind the cover plate, not actually "through" the box) and it is easy enough to disconnect the wires.