[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