koordlet: remove useless stateInformer.impl StateInformer interface

[j4ckstraw]

Ⅰ. Describe what this PR does

clean up stateInformer.impl StateInformer interface
use api.StateInformer interface instead

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

V. Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in make test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.04%. Comparing base (13a7107) to head (87bdcdd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2286      +/-   ##
==========================================
- Coverage   66.04%   66.04%   -0.01%     
==========================================
  Files         454      454              
  Lines       53438    53438              
==========================================
- Hits        35295    35292       -3     
- Misses      15601    15605       +4     
+ Partials     2542     2541       -1     

Flag	Coverage Δ
unittests	66.04% <100.00%> (-0.01%)	⬇️

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[saintube]

/lgtm