Remove duplicated code to evict entries. (#14410)

This code was factored out to a method, but also left in-place.

Calling this twice in a row makes no sense: the first call will reduce
the size appropriately, but the loop will immediately exit since the
cache size was already reduced.

changelog.d/14410.misc

@@ -0,0 +1 @@
+Remove unreachable code.

synapse/util/caches/stream_change_cache.py

@@ -72,7 +72,7 @@ class StreamChangeCache:
         items from the cache.
 
         Returns:
-            bool: Whether the cache changed size or not.
+            Whether the cache changed size or not.
         """
         new_size = math.floor(self._original_max_size * factor)
         if new_size != self._max_size:
@@ -188,14 +188,8 @@ class StreamChangeCache:
         self._entity_to_key[entity] = stream_pos
         self._evict()
 
-        # if the cache is too big, remove entries
-        while len(self._cache) > self._max_size:
-            k, r = self._cache.popitem(0)
-            self._earliest_known_stream_pos = max(k, self._earliest_known_stream_pos)
-            for entity in r:
-                del self._entity_to_key[entity]
-
     def _evict(self) -> None:
+        # if the cache is too big, remove entries
         while len(self._cache) > self._max_size:
             k, r = self._cache.popitem(0)
             self._earliest_known_stream_pos = max(k, self._earliest_known_stream_pos)
@@ -203,7 +197,6 @@ class StreamChangeCache:
                 self._entity_to_key.pop(entity, None)
 
     def get_max_pos_of_last_change(self, entity: EntityType) -> int:
-
         """Returns an upper bound of the stream id of the last change to an
         entity.
         """