remove hardcoded background color from ZoomControl

[axelpale]

The hardcoded background color is annoying if the client wants round button corners. The background then shows through. It cannot be overridden in stylesheets because it is element-level style, possibly being an artifact from an earlier development stage.

Currently the workaround is: zoomControl.element.style.backgroundColor = 'transparent'

The better solution is to remove the element-level styling completely.