Forward __getitem__ directly

All other methods are just forwarding operations to the underlying
`dict`, so there is no point in handling `KeyError`s manually in this
method.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/channels/experimental/_grouping_latest_value_cache.py

@@ -173,12 +173,7 @@ def __getitem__(self, key: HashableT) -> ValueT_co:
 
         Returns:
             The latest value that has been received for that key.
-
-        Raises:
-            KeyError: If no value has been received yet for that key.
         """
-        if key not in self._latest_value_by_key:
-            raise KeyError(f"No value received for key: {key!r}")
         return self._latest_value_by_key[key]
 
     @override