tor-urlbar-button-plain hover styling is overwritten by tor-button rule

In the recent changes to tor-colors.css I think I overrode the tor-urlbar-button-plain rules with a higher specificity selector.

E.g. the "Connect" urlbar button after a failed bootstrap should be grey, but it becomes purple when hovered.

To resolve this, I think it would be easiest to just remove the tor-button class when we use the tor-urlbar-button-plain class.