[avalanche] Remove the deprecatedrpc=getstakingreward option

Summary:
This has been deprecated for months and can now be removed.

Depends on D16365.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16366