UnknownDeviceDialog: get devices from SDK

rather than having to have the error message passed in.

This is in preparation for not having the dialog pop up straight
away when a message isn't sent so we don't have to keep the error
object knocking around somewhere.

src/UnknownDeviceErrorHandler.js

@@ -25,7 +25,6 @@ const onAction = function(payload) {
         const UnknownDeviceDialog = sdk.getComponent('dialogs.UnknownDeviceDialog');
         isDialogOpen = true;
         Modal.createTrackedDialog('Unknown Device Error', '', UnknownDeviceDialog, {
-            devices: payload.err.devices,
             room: payload.room,
             onFinished: (r) => {
                 isDialogOpen = false;