Revert "Allow long atoms (not sure if this is the way to do it)"

It is not.  You cannot generally do that from a thread as this is
not a module property.  Anyway, SWI-Prolog is already more permissive
than the ISO standard, which demands any newlines to be preceeded by a \

This reverts commit 5f615208a98a829f64de6dc21af492aff6e48a96.