Remove cereal submodule (pytorch#12666)

Summary:
Cereal is dead!

soumith orionr
Pull Request resolved: pytorch#12666

Reviewed By: soumith

Differential Revision: D10422061

Pulled By: goldsborough

fbshipit-source-id: ca1ac66d05e699df9de00fc340a399571b7ecb9f

.gitmodules

@@ -61,9 +61,6 @@
 [submodule "third_party/onnx"]
 	path = third_party/onnx
 	url = https://github.com/onnx/onnx.git
-[submodule "third_party/cereal"]
-	path = third_party/cereal
-	url = https://github.com/USCiLab/cereal
 [submodule "third_party/onnx-tensorrt"]
 	path = third_party/onnx-tensorrt
 	url = https://github.com/onnx/onnx-tensorrt