remove client4/client6 and anything using them

[rminnich]

Remove long-deprecated client4/client6 packages that no longer build
on bsd or darwin. This is a long-standing problem.

Remove all things depending on them, which have no users we know of.

Signed-off-by: Ronald G. Minnich rminnich@gmail.com

[codecov]

Codecov Report

Merging #459 (c253ca7) into master (3c283ff) will increase coverage by 5.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
+ Coverage   67.42%   72.80%   +5.37%     
==========================================
  Files          90       83       -7     
  Lines        3758     3217     -541     
==========================================
- Hits         2534     2342     -192     
+ Misses       1053      723     -330     
+ Partials      171      152      -19     

Flag	Coverage Δ
integtests	∅ <ø> (∅)
unittests	72.80% <ø> (+5.37%)	⬆️

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[insomniacslk]

CC @tfg13 - we only depend on it for OSF. In order to merge this PR we should:

• migrate to nclient{6,4}
• test through our OSF pipeline
• release to our customers, and merge once confirmed that it's working

Also CC @BigValen as the owner of our internal code that uses this. I can help with the above steps if needed

[pmazzini]

Related pinterest/bender#39

[pmazzini]

For awareness, the netboot package being removed here is used in r-root.

https://github.com/u-root/u-root/blob/main/cmds/boot/fbnetboot/main.go#L28