refactor: Remove global argument.current_realm.

zerver/lib/bugdown/arguments.py

@@ -1,9 +1,5 @@
 from typing import Any, Dict, Optional
 
-from zerver.models import Message, Realm
-
-current_realm = None  # type: Optional[Realm]
-
 # We avoid doing DB queries in our markdown thread to avoid the overhead of
 # opening a new DB connection. These connections tend to live longer than the
 # threads themselves, as well.