Remove onnx-simplify source code(deprecated)

[QingChuanWS]

1. The purpose of this PR: The onnx-simplify 3rd party package already supports importing custom operator.
2. For the repo that is still using the onnx simplify function in mmcv.onnx, firstly you need to install the onnx-simplify 3rd party package with the version greater than 0.3.0, then remove the mmcv.onnx.simplify and implement the onnx simplify according to the simplify() in onnxsim.
3. This PR is associated with mmdetection #4913, and mmcv.onnx.simplify is used in mmdetection to complete the simplify of the onnx model.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #927 (93971c2) into master (83cf25b) will increase coverage by 0.08%.
The diff coverage is n/a.

❗ Current head 93971c2 differs from pull request most recent head 0e43b15. Consider uploading reports for the commit 0e43b15 to get more accurate results

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
+ Coverage   64.62%   64.71%   +0.08%     
==========================================
  Files         148      145       -3     
  Lines        9354     9088     -266     
  Branches     1698     1626      -72     
==========================================
- Hits         6045     5881     -164     
+ Misses       2962     2889      -73     
+ Partials      347      318      -29     

Flag	Coverage Δ
unittests	64.71% <ø> (+0.08%)	⬆️

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Impacted Files	Coverage Δ
mmcv/cnn/bricks/conv.py	100.00% <ø> (ø)

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[RunningLeon]

This PR focus on remove the custom simplify model in mmcv.

@QingChuanWS
Please update the description of this PR:

1. clarify why we have to remove this feature
2. what impact it would have

[RunningLeon]

LGTM