[avalanche] Deprecate the -enableavalancheproofreplacement flag

Summary:
Make it always true, there is no point disabling that feature anymore after the remaining tests are updated to manage the replacement.

Depends on D11915 and D11919.

Test Plan:

ninja all check-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11921