[avalanche] Cleanup redundant argument in getavalancheinfo test

Summary: avalanche=1 is already set in extra_args

Test Plan:

ninja
./test/functional/test_runner.py abc_rpc_getavalancheinfo

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12077