PYTHON-3679 Fix Unused "type: ignore" comment

Author: ShaneHarvey

bson/son.py

@@ -115,7 +115,7 @@ def clear(self) -> None:
         self.__keys = []
         super(SON, self).clear()
 
-    def setdefault(self, key: _Key, default: _Value) -> _Value:  # type: ignore[override]
+    def setdefault(self, key: _Key, default: _Value) -> _Value:
         try:
             return self[key]
         except KeyError: