[avalanche] Add log message when dangling proofs get dropped

Summary:
Currently, logs for dangling-proof only appear when a proof is rejected for having already been dropped.
This is confusing because the initial dropping is not logged and may have occurred much earlier.

Test Plan:

ninja check bitcoind bitcoin-cli
./test/functional/test_runner.py abc_feature_proof_cleanup

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11880