QWlSurface: Remove unused static.

This went away in the buffer queue removal in
666f597ccd2ced23ecc71ba497981d1a88c34d77.

Change-Id: I9706f52026eb10e4ec17290262dfbfe52c14719f
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>