Drop unused 'io' module logic

[justsmth]

Issues:

N/A

Description of changes:

• Drop the unused and undocumented io modules types.

Call-outs:

• Ring 0.16 had an exposed, but undocumented, io::der module: https://docs.rs/ring/0.16.20/src/ring/io.rs.html#17
• The der module is being used by Rustls

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cc9f7ee) 95.82% compared to head (fcbbcc3) 95.83%.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #338    +/-   ##
========================================
  Coverage   95.82%   95.83%            
========================================
  Files          62       60     -2     
  Lines        8025     7868   -157     
========================================
- Hits         7690     7540   -150     
+ Misses        335      328     -7     

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