Remove unused access to addpeeraddress RPC output dict element

Summary: The ["success"] is unused and causing confusion. It may indicate a missing assert which is not the case here as some addresses are expected to fail, and the total number is asserted after the loop.

Test Plan:

./test/functional/test_runner.py abc-cmdline

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10924