Remove mention of a defunct hidden setting

Also removed the setting from the allowlist - this in theory can break
apps that somehow ended up using it. I expect there are no such apps and nobody is using it, since it doesn't have @UnsupportedAppUsage, so to use it someone would actually have to copy it from the code.

Bug: 331750107
Change-Id: I8b3fb97f85075ea4faae90653bb10744318072dc
Test: TH