caches: Remove unnecessary "in-memory" cache.

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.

zproject/computed_settings.py

@@ -361,9 +361,6 @@ CACHES = {
             "CULL_FREQUENCY": 10,
         },
     },
-    "in-memory": {
-        "BACKEND": "django.core.cache.backends.locmem.LocMemCache",
-    },
 }
 
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