remove ll_representation argument

[GertjanBisschop]

Removed argument of DemographicEvent.get_ll_representation(). Solves #1037 as this is no longer used by stdpopsim.
The TestLowLevelRepresentation test suite is marked as containing tests that were only needed to ensure compatibility with stdpopsim. I couldn't see any calls containing the optional argument. Are there any other tests I can remove @jeromekelleher?

[codecov]

Codecov Report

Merging #2231 (7db7dce) into main (11615e0) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2231   +/-   ##
=======================================
  Coverage   91.52%   91.52%           
=======================================
  Files          20       20           
  Lines       11334    11334           
  Branches     2302     2302           
=======================================
  Hits        10373    10373           
  Misses        523      523           
  Partials      438      438           

Flag	Coverage Δ
C	91.52% <100.00%> (ø)
python	98.70% <100.00%> (ø)

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Files	Coverage Δ
msprime/demography.py	99.33% <100.00%> (ø)

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[jeromekelleher]

I think it should be fine... I guess someone with an old stdpopsim who keeps their msprime up to date will hit problems?

Stdpopsim 0.2.0 came out over a year ago, but maybe the potential headaches aren't actually worth the small cleanup here.

https://github.com/popsim-consortium/stdpopsim/releases/tag/0.2.0

Let's keep this PR open and merge it in another year!