If you control-click on "Recently Bookmarked", and then click on "Delete" in the contextual menu, "Recently Bookmarked" will disappear...which is not to say that it won't reappear the next time you add a bookmark.
For me the Recently Bookmarked is greyed out and therefore inaccessible. If I try to do as you suggest, Recently Bookmarked doesn't even show up under Show All Bookmarks.
I'm actually using Firefox ESR 52.5.3 which according to Firefox is totally up-to-date.
[I just changed the thread's title.]