rpc: Remove special case for unknown service flags

Summary:
Backport of core PR19112.

Depends on D8519.

Test Plan:

./test/functional/test_runner.py rpc_net

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8526