fix: remove metadata field on StoreMessage (#135)

This field is a duplicate of `StoreContent.metadata` and is unsafe to
use as it is not part of the signed payload. This should not cause
issues as the field is not stored in DB on CCNs anyway.

Author: odesenfans

aleph_message/models/__init__.py

@@ -295,7 +295,6 @@ class StoreMessage(BaseMessage):
     type: Literal[MessageType.store]
     content: StoreContent
     forgotten_by: Optional[list] = None
-    metadata: Optional[Dict[str, Any]] = None
 
 
 class ForgetMessage(BaseMessage):