[avalanche] remove StakeSigner class

Summary:
This class was introduced for the legacy proof format because the stakes could not be signed before the proof data was complete. With the new proof format, each stake can be signed independently of other stakes, so the ProofBuilder can directly store the SignedStakes

Depends on D11914

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11916