remove useless code in groupchat; simplify the handling of termination msg

[sonichi]

Why are these changes needed?

• Removing an if condition that's never True.
• Include termination message for every agent's conversation history to keep it simple.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1abbcf3) 34.26% compared to head (d21f97e) 46.58%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1532       +/-   ##
===========================================
+ Coverage   34.26%   46.58%   +12.32%     
===========================================
  Files          42       42               
  Lines        5102     5098        -4     
  Branches     1167     1234       +67     
===========================================
+ Hits         1748     2375      +627     
+ Misses       3210     2523      -687     
- Partials      144      200       +56     

Flag	Coverage Δ
unittests	46.54% <100.00%> (+12.28%)	⬆️

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[afourney]

Looks good.

Not sure why it was ever doing that check around line #406

[sonichi]

Looks good.

Not sure why it was ever doing that check around line #406

I also changed the way to deal with termination message. It is broadcast to every other agent before exiting the chat. Then all the agents have the same messages in their conversation history. I think it simplifies things. Please let me know if this breaks anything for you.

[afourney]

Yeah saw that. I agree this is a better way to handle it. I am not aware of any scenarios this would harm.