Revert "Add a global instance for the config manager"

It seems to be more clear and easier to test to always pass a config
manager explicitly, so we just remove the global instance, which was
also a bit confusing when presented.

This reverts commit 77295dc.

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

Author: llucax

src/frequenz/sdk/config/__init__.py

@@ -3,11 +3,6 @@
 
 """Configuration management."""
 
-from ._global import (
-    get_config_manager,
-    initialize_config_manager,
-    shutdown_config_manager,
-)
 from ._logging_actor import LoggerConfig, LoggingConfig, LoggingConfigUpdatingActor
 from ._manager import ConfigManager
 from ._managing_actor import ConfigManagingActor
@@ -19,8 +14,5 @@
     "LoggerConfig",
     "LoggingConfig",
     "LoggingConfigUpdatingActor",
-    "get_config_manager",
-    "initialize_config_manager",
     "load_config",
-    "shutdown_config_manager",
 ]