[avalanche] Don't include validation.h via proof.h
Summary:
Lots of test files rely on this indirect inclusion which is bad. Remove the header from proof.h and update accordingly.
Depends on D17837.
Test Plan:
ninja all check
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17838