Delete PartialBeaconState (sigp#8591)

While reviewing Gloas I noticed we were updating `PartialBeaconState`. This code isn't used since v7.1.0 introduced hdiffs, so we can delete it and stop maintaining it 🎉

Similarly the `chunked_vector`/`chunked_iter` code can also go!


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

Co-Authored-By: Pawan Dhananjay <pawandhananjay@gmail.com>

Author: michaelsproul