edit_history: Remove `LEGACY_PREV_TOPIC` constant from code base.

Removes `LEGACY_PREV_TOPIC` which is no longer needed due to the
message edit history migration.

Also remove additions to the linter exclude list that were added
earlier in this commit series.

tools/linter_lib/custom_check.py

@@ -12,10 +12,7 @@ from zulint.custom_rules import Rule, RuleList
 
 FILES_WITH_LEGACY_SUBJECT = {
     # This basically requires a big DB migration:
-    "zerver/lib/message.py",
     "zerver/lib/topic.py",
-    "zerver/lib/types.py",
-    "zerver/views/message_edit.py",
     # This is for backward compatibility.
     "zerver/tests/test_legacy_subject.py",
     # Other migration-related changes require extreme care.
@@ -232,7 +229,7 @@ python_rules = RuleList(
     rules=[
         {
             "pattern": "subject|SUBJECT",
-            "exclude_pattern": "subject to the|email|outbox|edit_history_event",
+            "exclude_pattern": "subject to the|email|outbox",
             "description": "avoid subject as a var",
             "good_lines": ["topic_name"],
             "bad_lines": ['subject="foo"', " MAX_SUBJECT_LEN"],

zerver/lib/topic.py

@@ -20,12 +20,6 @@ MATCH_TOPIC = "match_subject"
 # Prefix use to mark topic as resolved.
 RESOLVED_TOPIC_PREFIX = "✔ "
 
-# This constant is actually embedded into
-# the JSON data for message edit history,
-# so we'll always need to handle legacy data
-# unless we do a pretty tricky migration.
-LEGACY_PREV_TOPIC = "prev_subject"
-
 # This constant is pretty closely coupled to the
 # database, but it's the JSON field.
 EXPORT_TOPIC_NAME = "subject"