cleanup: Remove test net support.

[iphydf]

Mainnet now supports NGC.

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This change is 

[JFreegman]

Are we never going to need this again?

[iphydf]

Are we never going to need this again?

Maybe we'll need something like this again, but we can add it back (in a different way) at that point. Right now it's a maintenance burden, every time having to read the correct enum when looking at packets during debugging.

[Green-Sky]

Should we allow different networks entirely? I think some dht networks use a simple passphrase.

[iphydf]

Should we allow different networks entirely? I think some dht networks use a simple passphrase.

Yes, I think that's a good idea. I think it can be done in a backwards compatible way (accepting both with and without network id).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2b0dc0f) 69.81% compared to head (65d09c9) 69.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2474      +/-   ##
==========================================
+ Coverage   69.81%   69.95%   +0.14%     
==========================================
  Files          75       75              
  Lines       25964    25964              
==========================================
+ Hits        18127    18164      +37     
+ Misses       7837     7800      -37     

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