Remove conference sessions from core workflow messages

Remove conference sessions from core workflow messages

These cause notices is some circumstance and it seems to make more
sense to remove than support them.

They are only available to be configured when the Event Cart is in play so
it's not clear it makes sense to fix core code for them

The Event cart is kinda on it's way out & I don't think it makes sense
to maintain conference sessions in the core templates to support this
extension which despite many attempts has never had much use & has always
been somewhat broken AFAIK.

Note that this change would affect sites with non-customised templates
but in time I think we would either move the assign to the cart or stop
assigning it

Also not the goal is to provide event-cart type functionality via form
builder