'How do I delete the previously navigated-to websites from the URL window' - simple.
01. Quit 'Safari'.
02. Locate and open the 'Safari' folder ('~/Library/Safari/').
03. Delete the 'Bookmarks.plist' and 'History.plist' files.
04. Open (launch, run) '
TinkerTool', click on its 'Safari' icon button, set the 'History: Storage: Visible items in main history menu:' and 'Maximum number of stored items:' popup menus to 'None', and quit 'TinkerTool'.
05. Open (launch, run) 'Safari'.
06. Never add a URL to either the 'Bookmark Menu' or the 'Bookmark Bar'.
Once you add a Bookmark to either the 'Menu' or 'Bar', and / or maintain a 'history' of previously visited web sites - 'Safari' will fabricate the URL drop-down list of URL's (that you see as you enter a URL) based on the 'Bookmarks.plist' and / or History.plist' entries.