fix(prompt-optimizer): switch to built-in system prompt

- Replace the system prompt of the prompt optimization model with a built-in prompt.
- Remove system prompt entries from the database.
- Remove the API endpoint for managing system prompt configuration.

api/app/models/__init__.py

@@ -20,7 +20,7 @@ from .data_config_model import DataConfig
 from .multi_agent_model import MultiAgentConfig, AgentInvocation
 from .workflow_model import WorkflowConfig, WorkflowExecution, WorkflowNodeExecution
 from .retrieval_info import RetrievalInfo
-from .prompt_optimizer_model import PromptOptimizerModelConfig, PromptOptimizerSession, PromptOptimizerSessionHistory
+from .prompt_optimizer_model import PromptOptimizerSession, PromptOptimizerSessionHistory
 
 __all__ = [
     "Tenants",
@@ -56,7 +56,6 @@ __all__ = [
     "WorkflowExecution",
     "WorkflowNodeExecution",
     "RetrievalInfo",
-    "PromptOptimizerModelConfig",
     "PromptOptimizerSession",
     "PromptOptimizerSessionHistory"
 ]