How do I remove this shower fixture?
It looks like the screw in the center is missing (the hole in the center of the piece that says "off").  I would try pulling on that center part to remove it (the screw that was there probably was to hold it in place but now that it is gone it should come off... easier said than done).  You will probably need to pull fairly hard or even use a screw driver to slowly pry it off (especially if it has been on for a long time it might take some work to get it off).  Then once that center piece comes off you should see the other screws that are holding the rest of the unit on.