What is this kind of security bolt head called, and how can I remove it?
It's called a spanner slotted (or slotted spanner) head. Useful site here. Hard to find in the UK, why the owner used one I cannot imagine. Screwfix doesn't have them. This site may do, although it is international. You'll need to figure out the correct size before you order. 

Alternatively get a real cheap flat-head screwdriver the right width and file down the middle until you get the right notch.

If you can grip it with a pliers try a pair of locking pliers and unscrewing it that way I keep a mini vise-grips handy for that sort of thing.