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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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