[avalanche] Cleanup invalidated dangling proofs

Summary:
Once placed in the dangling proof pool, proofs are not checked for validity.
Although they cannot be pulled back to become valid peers, it does not make sense to
hold onto them anymore.

This fix ensures dangling proofs are always valid.

Test Plan:

ninja check-avalanche

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16608