[avalanche] Cleanup redundant check for avalanche processor
Summary: D16313 refactored this check to no longer check gArgs for avalanche being enabled. This can be refactored further to remove the redundant check.
Test Plan:
ninja check check-functional
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, PiRK
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17171