Disable recent search suggestions

notes

• browser.urlbar.recentsearches.featureGate enables "trending suggestions" and "recent searches"
• recent searches is browser.urlbar.suggest.recentsearches
• meta https://bugzilla.mozilla.org/show_bug.cgi?id=1858386

I am not sure where this data is kept and how it is sanitized. It seems as if they are kept for up to 3 days (and sanitizing history does not remove them?)