Removed functions code && supported functions plugin

[klakhov]

Motivation and context

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into the CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)
• I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov]

Codecov Report

Merging #5979 (7b4d5f3) into develop (1ef6ba0) will increase coverage by 0.17%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           develop    #5979      +/-   ##
===========================================
+ Coverage    82.39%   82.56%   +0.17%     
===========================================
  Files          362      362              
  Lines        39405    39264     -141     
  Branches      3592     3578      -14     
===========================================
- Hits         32467    32419      -48     
+ Misses        6938     6845      -93     

Components	Coverage Δ
cvat-ui	77.71% <66.66%> (+0.31%)	⬆️
cvat-server	86.96% <ø> (+<0.01%)	⬆️

[bsekachev]

LGTM, but postpone merge, because I need first to look at the second part