[avalanche] Cleanup deprecated 'sharing' RPC field

Summary: This has been deprecated for 6 months, let's remove it.

Test Plan:

./test/functional/test_runner.py rpc_deprecated

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15991