Remove outdated sections for Network Buffers and Async Checkpoints from the Large State Tuning Guide

The documentation page "Tuning Checkpoints and Large State" has a section on network buffers and async checkpoints.

The network buffers section is not relevant any more and say so as well (before Flink 1.3 ...)

The async snapshots section is also outdated (all state snapshots are async by default now) refers to a setup (rocks with heap timers) that is no longer the default and has its own warning already.