Set Dock border to none, to fix position consistency of atom-dock.

The dock has 0px border in Desktop, but "transparent" border in Overview screen, meaning it still have a width. This leads to position inconsistency between the dock in desktop and in overview (I use height as a variable to determine it's y position, 1px border will make the dock shift 1px).
Because on overview screen, the background and border of dock is supposed to be transparent after all, I set the borded to none.