Removed ServerBenchmark

Given that prune now is testing performance quite well, and appears to be able to reliably pick up and identify regressions in many places, and also given that we we haven't run the benchmark in a long time, I've removed the ServerBenchmark to simplify the build.

Also removed the NettyHandlerSpec, which depended on the same NettyRunner that the ServerBenchmark depended on - all the tests in there were covered by the other tests that made real HTTP connections to the server, and so didn't just test that we were calling what we thought we were supposed to be calling on Netty, but tested that we were calling the right methods on Netty.

The reason I've deleted these is because they were getting quite expensive to maintain - there was a custom implementation of a router, custom implementation of play.api.Application, etc, and I was finding in the implementation of many of the DI changes in Play, there was always something in these benchmarks that was breaking that needed to be fixed, because these tests weren't reusing any of the existing test infrastructure provided by Play.