gh-136021: Remove dead code for internal sentinel in typing (#138120)

Remove dead code for internal sentinel

Author: brianschubert

Lib/typing.py

@@ -453,12 +453,6 @@ def _deprecation_warning_for_no_type_params_passed(funcname: str) -> None:
     warnings.warn(depr_message, category=DeprecationWarning, stacklevel=3)
 
 
-class _Sentinel:
-    __slots__ = ()
-    def __repr__(self):
-        return '<sentinel>'
-
-
 def _eval_type(t, globalns, localns, type_params, *, recursive_guard=frozenset(),
                format=None, owner=None, parent_fwdref=None):
     """Evaluate all forward references in the given type t.