Remove broken Commmand Line guide link

The Ruby on Rails Guides Guide recommends:
  > To generate all the guides, just cd into the guides directory, run bundle install and execute: rake guides:generate
  > It is also recommended that you work with WARNINGS=1. This detects duplicate IDs and warns about broken    internal links.

Currently, on master, that produces one warning:

```
Generating command_line.md as command_line.html
*** BROKEN LINK: #server-with-different-backends, perhaps you meant #rails-with-databases-and-scm.
```

This PR removes that broken link. I have removed the entire line, because the section it is referring to has been removed (there is no discussion, as far as I can tell, in any Rails Guides about webservers other than Webrick.)