remove obsolete CSS class from upload form message-block

I removed this class because it is messing up my custom theme. It is obsolete because all possible child nodes have their own _»message«_ class __.g-error__.

I would love to see this class disappear because it looks wrong or at least misused here to me. There are no styling effects to the default theme by removing it because all rules are overridden by the child node rules anyway.

kind regards,
Moritz