log: Remove "No rpcpassword set" from logs

Summary:

rpcpassword is deprecated and not recommended anymore. So remove it from the logs, which indicate that an rpcpassword should be set and cause confusion.

This is a backport of Core PR18999

Test Plan: src/bitcoind -regtest -printtoconsole | grep cookie

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9137