Removing "specific use cases" section

This section was incorrect because it assumed that the header was not already present in the reference set.  (In that case, indexing an entry three times emits the header just once.)

More fundamentally, there just isn't a good reason to do this.  There might be value in providing multiple values for the same header, but repeating values is just odd.

I'm sure that people will be able to work this out if they need to do it.  I'd be more interested in providing guidance on how to optimally emit different header fields, say Cache-Control, where you might use multiple header table entries indexed selectively, to get different combinatorial effects without ever requiring any literals.