Remove the `Id` class from mypy_extensions (#5441)

Argh, I forgot to remove this when doing cleanup.
diff --git a/mypy_extensions.py b/mypy_extensions.py
index a05ea55..c5442a3 100644
--- a/mypy_extensions.py
+++ b/mypy_extensions.py
@@ -157,5 +157,3 @@
 
 
 FlexibleAlias = _FlexibleAliasCls()
-
-Id = _FlexibleAliasCls()