Remove extraneous check on encoded data length (#5896)
Remove extraneous check on encoded data length (#5896)

The encoded data length is something that can change over time with
improvements to the underlying compression. We trust the underlying
`encode_op`, so just checking that that we have a valid PNG header is
sufficient.
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard