chore(prompts): remove duplicate variable definition dead code

[stevenaldinger]

Summary

PROMPT_EXTRACT_BLOCKS variable was defined twice, so the first definition was immediately overwritten.

List of files changed and why

crawl4ai/prompts.py - to remove first PROMPT_EXTRACT_BLOCKS variable definition that was not doing anything because it was immediately overwritten

How Has This Been Tested?

This is a chore commit that had no functional effect.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes