message: Get rid of unwanted deprecation warning in optimizer v2.
  Currently, optimizers need to access variable.constraint. Doing so raises a user-visible deprecation warning. As a result, every time an optimizer is used in TF 2.0, a deprecation warning is raised, which is not an acceptable user-facing behavior.
diff: @@ -540,9 +540,6 @@ class BaseResourceVariable(variables.VariableV1):
  return self._initial_value
  @property
  - @deprecated(
  - None,
  - "Apply a constraint manually following the optimizer update step.")
  def constraint(self):
  """Returns the constraint function associated with this variable.
  