update docs about onnx-simplify

docs/onnx.md

@@ -14,36 +14,6 @@ opset_version = 11
 register_extra_symbolics(opset_version)
 ```
 
-## ONNX simplify
-
-### Intention
-
-`mmcv.onnx.simplify` is based on [onnx-simplifier](https://github.com/daquexian/onnx-simplifier), which is a useful tool to make exported ONNX models slimmer by performing a series of optimization. However, for Pytorch models with custom op from `mmcv`, it would break down. Thus, custom ops for ONNX Runtime should be registered.
-
-### Prerequisite
-
-`mmcv.onnx.simplify` has three dependencies: `onnx`, `onnxoptimizer`, `onnxruntime`. After installation of `mmcv`, you have to install them manually using pip.
-
-```bash
-pip install onnx onnxoptimizer onnxruntime
-```
-
-### Usage
-
-```python
-import onnx
-import numpy as np
-
-import mmcv
-from mmcv.onnx.simplify import simplify
-
-dummy_input = np.random.randn(1, 3, 224, 224).astype(np.float32)
-input = {'input':dummy_input}
-input_file = 'sample.onnx'
-output_file = 'slim.onnx'
-model = simplify(input_file, [input], output_file)
-```
-
 ### FAQs
 
 - None