Disable centre panel override on bottom taskbar

If you're using the taskbar on the bottom of the screen, there won't be a centre panel, so there's no reason to compensate for it.

I'm pretty sure that's true, anyway. There's not any way to have a centre panel on the bottom taskbar, is there? Obviously, if there is, you should probably reject this pull request (though I'd love to know how to disable this invisible centre panel in my own bottom taskbar :P).

(Is this true for the right panel as well? I'm honestly not sure. If so, you'd also want to change line 1361 to remove the subtraction; I've done this in my local setup.)