Doc: remove text about txes always relayed from -whitelist

Summary:
Updates text since -whitelistforcerelay was set to false by default in
PR15193.

Backport of Core PR15890

Test Plan: ninja && src/bitcoind -help

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7975