fix: RolePlaying model setting would be override by default model

[Wendong-Fan]

Description

remove default self.model setting in RolePlaying, as the empty dict passed to ChatAgent would use the default model set in ChatAgent side

Motivation and Context

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Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of example)

Implemented Tasks

• Subtask 1
• Subtask 2
• Subtask 3

Checklist

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• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.