Docker: Stop installing cudnn

It is already installed on kaixhin/cuda-torch and latest cudnn does not work with old cunn.

Dockerfile

@@ -15,8 +15,7 @@ RUN \
   luarocks install md5 && \
   luarocks install uuid && \
   luarocks install csvigo && \
-  PREFIX=$HOME/torch/install luarocks install turbo && \
-  luarocks install cudnn
+  PREFIX=$HOME/torch/install luarocks install turbo
 
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