[avalanche] Remove unused hash verifier in avahello deserialization

Summary: This has never been used and compute a hash of the message for no reason. There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11722