remove legacy plugins

[Borda]

What does this PR do?

removing legacy plugins, follow up to #5743
cc: @awaelchli
there are some lining to accelerators so it would be better to go after #5949

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[pep8speaks]

Hello @Borda! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-02-16 18:03:04 UTC

[codecov]

Codecov Report

Merging #5950 (b6c3575) into master (fcfa7fa) will increase coverage by 3%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #5950    +/-   ##
=======================================
+ Coverage      90%     93%    +3%     
=======================================
  Files         170     159    -11     
  Lines       11784   11093   -691     
=======================================
- Hits        10664   10318   -346     
+ Misses       1120     775   -345     

[tchaton]

LGTM !