[avalanche] Remove dead test code in the proof builder

Summary:
The buildRandom method is no longer used. Specialized functions are used in tests instead.

Ref T1676.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10078