link.removeAttr(path [, opt])
Remove a previously set attribute from the link.
path can either be a string that specifies the path to the (possibly nested) attribute to be removed, or an array of more paths. The associated link view makes sure the link gets re-rendered properly.
opt is passed, it can contain data that is passed over to the event listeners for the
change:attrs event triggered on the link itself and also on the graph the link is in.