Remove incorrect type annotation.

PiperOrigin-RevId: 404338838

Author: MarkDaoust

tools/tensorflow_docs/api_generator/parser.py

@@ -1824,8 +1824,8 @@ def __init__(self, *, full_name: str, py_object: Any, **kwargs) -> None:
     self._signature = None
 
   @property
-  def signature(self) -> None:
-    return self._signature  # pytype: disable=bad-return-type  # bind-properties
+  def signature(self):
+    return self._signature
 
   def _custom_join(self, args: List[str], origin: str) -> str:
     """Custom join for Callable and other type hints.