remove inappropriate api_type

[sonichi]

Why are these changes needed?

When the api type is openai, the key "api_type" is not supposed to appear in the config.

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 (083ba32) 30.38% compared to head (fc1e67c) 49.72%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1190       +/-   ##
===========================================
+ Coverage   30.38%   49.72%   +19.33%     
===========================================
  Files          32       32               
  Lines        4302     4302               
  Branches      994     1045       +51     
===========================================
+ Hits         1307     2139      +832     
+ Misses       2901     1959      -942     
- Partials       94      204      +110     

Flag	Coverage Δ
unittests	49.62% <ø> (+19.29%)	⬆️

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