Remove extra parameter in DemographicEvent.get_ll_representation

[jeromekelleher]

We have an extra unused parameter in the get_ll_representation for each DemographicEvent class. This is needed because stdpopsim will uses this until msprime 1.0 is released. Stdpopsim should then transition to using the richer API from msprime instead of doing things locally.

https://github.com/popsim-consortium/stdpopsim/issues/535

Once stdpopsim 0.2.0 has been released using this new API it should be safe to remove the unused parameters, as the stdpopsim usage is a development tool rather than something used directly in simulations.

See #1010

[jeromekelleher]

Can you check if this can be done now @GertjanBisschop? If stdpopsim has been using msprime >= 1.0 for a while then I think we can clean this up.

[jeromekelleher]

Adding to 1.3.1 milestone for now.