style(proxy): drop the explicit return None update_database no longer needs

Reverting the function to -> None left two bare `return None` statements that
RET501 rejects now that None is the only value it can return.

Claude-Session: https://claude.ai/code/session_01QvQzYztinxj8ZuD5YxbVdL

litellm/proxy/db/db_spend_update_writer.py

@@ -232,7 +232,7 @@ class DBSpendUpdateWriter:
                 team_id,
             )
             if ProxyUpdateSpend.disable_spend_updates() is True:
-                return None
+                return
             if token is not None and isinstance(token, str) and token.startswith("sk-"):
                 hashed_token = hash_token(token=token)
             else:
@@ -318,7 +318,7 @@ class DBSpendUpdateWriter:
                 org_id,
                 end_user_id,
             )
-            return None
+            return
 
     async def _enqueue_tool_usage_transaction(
         self,