Revert "Add a Reconfigurable *mixin*"

This class proved to be more confusing than helpful when presented to
a wider audience, it seems to be better to propose ways to implement
actors as documentation.

This reverts commit 3201348.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/config/__init__.py

@@ -11,7 +11,6 @@
 from ._logging_actor import LoggerConfig, LoggingConfig, LoggingConfigUpdatingActor
 from ._manager import ConfigManager
 from ._managing_actor import ConfigManagingActor
-from ._reconfigurable import Reconfigurable
 from ._util import load_config
 
 __all__ = [
@@ -20,7 +19,6 @@
     "LoggerConfig",
     "LoggingConfig",
     "LoggingConfigUpdatingActor",
-    "Reconfigurable",
     "get_config_manager",
     "initialize_config_manager",
     "load_config",