Remove above-ndb memcaching

[BenHenning]

Per https://cloud.google.com/appengine/docs/standard/python/ndb/cache, NDB performs both in-context and NDB caching for all reads and writes, and does auto-batching for group operations. This is enabled by default, and likely leads to our own memcache operations being completely redundant. Also, NDB properly handles race conditions that can arise when reading and writing to memcache without compare-and-swap operations, which Oppia doesn't currently do.

To summarize, removing our memcaching will likely result in:

1. Less race conditions since we aren't handling memcache race conditions
2. More performant caching since NDB includes in-context caching
3. Simpler code since working with memcache is complex

We can still use memcache for caching outputs of expensive computations, but I don't think we do that anywhere (we generally rely on NDB for this purpose).

[BenHenning]

/cc @seanlip

[BenHenning]

One other benefit: NDB properly handles memcache updates within transactions (whereas we might not be doing this correctly now, but we rarely use transactions so I doubt we actually see bugs from this--however we should be using more transactions so this could become a worse problem over time).