Remove keylen from LruCache. (#9993)

`keylen` seems to be a thing that is frequently incorrectly set, and we don't really need it.

The only time it was used was to figure out if we had removed a subtree in `del_multi`, which we can do better by changing `TreeCache.pop` to return a different type (`TreeCacheNode`).

Commits should be independently reviewable.

synapse/util/caches/deferred_cache.py

@@ -70,7 +70,6 @@ class DeferredCache(Generic[KT, VT]):
         self,
         name: str,
         max_entries: int = 1000,
-        keylen: int = 1,
         tree: bool = False,
         iterable: bool = False,
         apply_cache_factor_from_config: bool = True,
@@ -101,7 +100,6 @@ class DeferredCache(Generic[KT, VT]):
         # a Deferred.
         self.cache = LruCache(
             max_size=max_entries,
-            keylen=keylen,
             cache_name=name,
             cache_type=cache_type,
             size_callback=(lambda d: len(d) or 1) if iterable else None,