Removed a setter for tf.TensorShape.dims

To the best of my knowledge it is not used anywhere internally and can
be safely removed. External users are encouraged to replace

  shape.dims = ...

with

  shape = tf.TensorShape(...)

PiperOrigin-RevId: 240592991

tensorflow/python/framework/tensor_shape.py

@@ -779,10 +779,6 @@ class TensorShape(object):
     """Returns a list of Dimensions, or None if the shape is unspecified."""
     return self._dims
 
-  @dims.setter
-  def dims(self, dims):
-    self._dims = dims
-
   @property
   def ndims(self):
     """Deprecated accessor for `rank`."""