Don't tint selected rooms black

[nadonomy]

Matthew:

The fact that your currently selected room in the RoomList goes black makes it look as it has unread messages, as we've trained people to see blank-label == unread.

[nadonomy]

Actioned in matrix-org/matrix-react-sdk#2378.

Of note:

During development, I lost confidence in #7891 making sense anymore while using the app with all these changes in chorus. I've left it in, to dogfood, and we can just revert commit 9b8f07c if need be.