Remove dead internal code

[destel]

Remove dead internal code: core.Delay and internal/ringbuffer.
These were never a part of the public api

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.57%. Comparing base (990a93f) to head (6644b3c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   99.03%   99.57%   +0.53%     
==========================================
  Files          17       15       -2     
  Lines         832      708     -124     
==========================================
- Hits          824      705     -119     
+ Misses          8        3       -5     

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