Remove info about DHT from Crypto section

[Diadlo]

I check the 'crypto_core' twice and not found functions which can be related with DHT

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This change is 

[CLAassistant]

All committers have signed the CLA.

[iphydf]

Make the change in https://github.com/TokTok/hs-toxcore, then we sync.

[iphydf]

Please enable the checkbox "Allow edits from maintainers." on the bottom right.

[Diadlo]

@iphydf Done