[avalanche] Clear finalization when manually switching chaintips

Summary:
Presumably calling park/unpark via RPC indicates intent to switch chaintips
even if the current tip has been avalanche finalized. Clearing this will allow
the retriggering of polling blocks near the tip(s).

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12885