[avalanche] Rename isRemoteProof to hasRemoteProofStatus

Summary: isRemoteProof is a misnomer because it does not check the remote status. This confusion led to incorrect usage in contenders which will be fixed in a near future diff.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18010