[avalanche] Do not poll avalanche invalidated blocks more than once

Summary:
Although rare it is possible to trigger polling of a block after avalanche has
concluded that it is invalidated. This is already not the case for finalized
blocks so make the behavior the same for invalidated blocks.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13207