Remove references to removed plugins

[pnuu]

This PR removes references to removed plugins from the documentation.

• Closes Documentation refers to non-existing plugins check_platform and check_sensor #98

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.96%. Comparing base (98355d9) to head (7aba442).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #221   +/-   ##
=======================================
  Coverage   95.96%   95.96%           
=======================================
  Files          14       14           
  Lines        3268     3268           
=======================================
  Hits         3136     3136           
  Misses        132      132           

Flag	Coverage Δ
unittests	95.96% <ø> (ø)

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[gerritholl]

Looks good to me. When were they removed? Replaced by check_metadata?