Re-enable system prompt override

[TamiTakamiya]

Description

System Prompt Override added with #227 is no longer working. System prompt was set using the
_system_prompt attribute of QueryHelper class. However, the DocsSummarizer class, which extends
QueryHelper, now has system_prompt attribute, which does not contain the logic required for
System Prompt Override. We can simply use the _system_prompt attribute as it is set in the constructor
of QueryHelper.

Type of change

• Refactor
• New feature
• Bug fix
• CVE fix
• Optimization
• Documentation Update
• Configuration Update
• Bump-up dependent library
• Bump-up library or tool used for development (does not change the final image)
• CI configuration change
• Konflux configuration change

Related Tickets & Documents

• Related Issue #
• Closes #

Checklist before requesting a review

• I have performed a self-review of my code.
• PR has passed all pre-merge test jobs.
• If it is a core feature, I have added thorough tests.

Testing

• Please provide detailed steps to perform tests related to this code change.
• How were the fix/results from this change verified? Please provide relevant screenshots or results.
  Ansible Lightspeed chatbot has the UI to override system prompt and it can be used for verification.