Remove disabled partial implementation of streaming $group

We have code leftover in the code base from a partial implementation of SERVER-4507 that was never completed. (The optimization described by SERVER-4507 would improve $group on a sorted input sequence to avoid constructing a hash table.) The streaming $group code is disabled and largely untested, so it has caused confusion and may develop bugs over time. This task for this ticket is to delete any dead code related to streaming $group. This code can be reintroduced in the future if SERVER-4507 is scheduled for work.