Clean up unused listings in Ch 12

[ekcom]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • no-listing-05-fix-worker-new
  • no-listing-06-fix-threadpool-drop
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • src/ch21-03-graceful-shutdown-and-cleanup.md

URL to the section(s) of the book with this problem: N/A but indirectly https://doc.rust-lang.org/book/ch21-03-graceful-shutdown-and-cleanup.html

Description of the problem:
Listings no-listing-05-fix-worker-new and no-listing-06-fix-threadpool-drop are not used in the chapter. In the Markdown source file, no-listing-04 is used, then the listings 21-23, 21-24, and 21-25, and then no-listing-07. Further, no-listing-05 has outdated code (as updated in #3248).

Suggested fix:
Remove the no-listing-05-fix-worker-new and no-listing-06-fix-threadpool-drop listings, potentially shifting the current no-listing-07 number to be no-listing-05.