Remove memcached support

Apparently we had memcached support. It was never worth it, and put
a weird md5 hash calculation in the path of regular caching. Seeing
as it was completely undocumented, I doubt anyone has ever used it.

overviewer.py

@@ -455,8 +455,6 @@ dir but you forgot to put quotes around the directory, since it contains spaces.
     # Set up the cache objects to use
     caches = []
     caches.append(cache.LRUCache(size=100))
-    if config.get("memcached_host", False):
-        caches.append(cache.Memcached(config['memcached_host']))
     # TODO: optionally more caching layers here
 
     renders = config['renders']