[avalanche] Clarify the processor includes
Summary: The CBlockIndex is now used in the comparator and is not forwarded, but indirectly included. The ProofRef is also indirectly included, while Proof is no longer used. This diff cleans these inclusions so it's easier to reason about.
Test Plan:
With Clang and GCC:
ninja all check-avalanche
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D13060