Remove system message functionality from code executors

[jackgerrits]

Why are these changes needed?

@ekzhu as discussed this removes all system message functionality. We agreed that this should be solved a different way, starting with documenting how to create good system messages. Doing this now before code executors become too mature.

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 ✅

Project coverage is 42.81%. Comparing base (b93e2c5) to head (cf1b5a6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1835      +/-   ##
==========================================
+ Coverage   37.33%   42.81%   +5.47%     
==========================================
  Files          64       64              
  Lines        6913     6879      -34     
  Branches     1519     1646     +127     
==========================================
+ Hits         2581     2945     +364     
+ Misses       4109     3688     -421     
- Partials      223      246      +23     

Flag	Coverage Δ
unittests	42.69% <ø> (+5.35%)	⬆️

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[sonichi]

Could you make the PR from the upstream to run the openai test?

[jackgerrits]

See PR: #1933