[avalanche] Simplify the updatedBlockTip handler

Summary:
We can use the rejectProof facility for newly invalidated proofs. This avoids a duplicate and makes the code more readable. There is no change in behavior.

Ref T1854.

Depends on D10711.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: tyler-smith

Subscribers: tyler-smith

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10712