This cache was added in da33b72848 to serve as a replacement for the
durable database cache, in development; the previous commit has
switched that to be the non-durable memcached backend.
The special-case for "in-memory" in development is mostly-unnecessary
in contrast to memcached -- `./tools/run-dev.py` flushes memcached on
every startup. This differs in behaviour slightly, in that if the
codepath is changed and `run-dev` restarts Django, the cache is not
cleared. This seems an unlikely occurrence, however, and the code
cleanup from its removal is worth it.
56058f3316