Remove some outdated description in readme

[hellock]

No description provided.

[codecov]

Codecov Report

Merging #4681 (723ab82) into master (44b4106) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4681      +/-   ##
==========================================
+ Coverage   64.01%   64.03%   +0.01%     
==========================================
  Files         251      251              
  Lines       19441    19441              
  Branches     3282     3282              
==========================================
+ Hits        12446    12449       +3     
- Misses       6334     6335       +1     
+ Partials      661      657       -4     

Flag	Coverage Δ
unittests	64.00% <ø> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/test_mixins.py	45.03% <0.00%> (-8.40%)	⬇️
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	73.72% <0.00%> (-1.88%)	⬇️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	100.00% <0.00%> (+5.37%)	⬆️
mmdet/models/roi_heads/mask_scoring_roi_head.py	87.50% <0.00%> (+32.14%)	⬆️

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