edit_history: Remove prev_subject from EditHistoryEvent type.

It is no longer possible for this field to appear in actual entries.

zerver/lib/message.py

@@ -514,7 +514,7 @@ class MessageDict:
                 if "prev_topic" in edit_history_event:
                     # The prev_subject field has been renamed in the
                     # database, but not the API.
-                    edit_history_event["prev_subject"] = edit_history_event["prev_topic"]
+                    edit_history_event["prev_subject"] = edit_history_event["prev_topic"]  # type: ignore # Temporary type-checking violation
 
                 # New fields not consistently available, and thus
                 # intentionally not exposed to the API.
@@ -525,7 +525,7 @@ class MessageDict:
                 if "topic" in edit_history_event:
                     del edit_history_event["topic"]
 
-                edit_history.append(edit_history_event)
+                edit_history.append(edit_history_event)  # type: ignore # Temporary type-checking violation
             obj["edit_history"] = edit_history
 
         if Message.need_to_render_content(

zerver/lib/types.py

@@ -121,9 +121,6 @@ class EditHistoryEvent(TypedDict, total=False):
     timestamp: int
     prev_stream: int
     stream: int
-    # One of prev_subject and prev_topic is guaranteed to exist; if
-    # both exist they will have identical values.
-    prev_subject: str
     prev_topic: str
     topic: str
     prev_content: str

zerver/views/message_edit.py

@@ -54,10 +54,6 @@ def fill_edit_history_entries(
         if "prev_topic" in edit_history_event:
             prev_topic = edit_history_event["prev_topic"]
             formatted_entry["prev_topic"] = prev_topic
-        elif LEGACY_PREV_TOPIC in edit_history_event:
-            # TODO: Delete this once we've finished migrating legacy message objects.
-            prev_topic = edit_history_event["prev_subject"]
-            formatted_entry["prev_topic"] = prev_topic
 
         # Fill current values for content/rendered_content.
         if "prev_content" in edit_history_event: