Leave out optional keys from /sync (#9919)

This leaves out all optional keys from /sync. This should be fine for all clients tested against conduit already, but it may break some clients, as such we should check, that at least most of them don't break horribly and maybe back out some of the individual changes. (We can probably always leave out groups for example, while the others may cause more issues.)

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>