refactor: remove unused is_deleted field from conversations table

Remove the is_deleted field that was never utilized for soft deletion.
This simplifies queries and reduces unnecessary database overhead.

Fixes #25017

api/models/model.py

@@ -745,8 +745,6 @@ class Conversation(Base):
         "MessageAnnotation", backref="conversation", lazy="select", passive_deletes="all"
     )
 
-    is_deleted: Mapped[bool] = mapped_column(sa.Boolean, nullable=False, server_default=sa.text("false"))
-
     @property
     def inputs(self) -> dict[str, Any]:
         inputs = self._inputs.copy()