The extension is called "Menu extension" and will disappear when you turn it off under "Shell Extensions" in the program Advanced Settings. Turning off the bottom panel extension only moves mint menu to the top panel.
If you want a default shell-ish experience, turn off bottom panel extension, window list extension and menu extension. The Smart Overview extension does more damage than good so I'd recommend that turned off as well.
Thank you for this thread. That’s all I can say. You most definitely have made this forum into something special. You clearly know what you are doing, you’ve covered so many bases. Thanks!