fix: don't import AsyncBoltAgent in agent __init__ to avoid aiohttp dependency

AsyncBoltAgent imports AsyncWebClient which requires aiohttp. Eagerly
importing it from the agent package __init__ breaks environments where
aiohttp is not installed, since slack_bolt/__init__.py imports BoltAgent
from this package. Follows the existing convention of not adding async
module imports at the top level.

Author: mwbrooks

slack_bolt/agent/__init__.py

@@ -1,7 +1,5 @@
 from slack_bolt.agent.agent import BoltAgent
-from slack_bolt.agent.async_agent import AsyncBoltAgent
 
 __all__ = [
-    "AsyncBoltAgent",
     "BoltAgent",
 ]