Remove out-of-date docs on new workers

In response to: https://github.com/alphagov/notifications-api/pull/3332#issuecomment-921897641

These were so out-of-date as to be misleading. We can always refer
back to them in the version history if needed.

README.md

@@ -120,5 +120,4 @@ All commands and command options have a --help command if you need more informat
 
 ## Further documentation
 
-- [Creating a new worker app](docs/creating-a-new-worker-app.md)
 - [Writing public APIs](docs/writing-public-apis.md)