This isn't used by anything in the tree and Piper doesn't use it either.
(`ratbagctl` queries it from the sodipodi name in the SVG).
Plus, assuming we run sandboxed the filename is useless anyway, `GetSvgFd()`
takes care of this bit though.
So let's drop this.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>