Removing GAN link (deprecated) (#8095)

Since the GAN link is now dead, it can safely be removed.

research/README.md

@@ -33,7 +33,6 @@ request.
 -   [domain_adaptation](domain_adaptation): domain separation networks.
 -   [fivo](fivo): filtering variational objectives for training generative
     sequence models.
--   [gan](gan): generative adversarial networks.
 -   [im2txt](im2txt): image-to-text neural network for image captioning.
 -   [inception](inception): deep convolutional networks for computer vision.
 -   [keypointnet](keypointnet): discovery of latent 3D keypoints via end-to-end