On macOS High Sierra I have install the "emacs-mac" Emacs port via homebrew. The Emacs version now is 25.3.3.
When starting up Emacs I get this strange tab in my Emacs frame:
I have set the Document tab setting "Prefer tabs when opening documents" in the Dock Preference pane to "Manually". I still have this useless tab.
How can I get rid of it?
A similar question has been posted and solved before here.
More info on the issue can be found under this Github ticket.
So, to answer your question, you can do M-x mac-toggle-tab-bar and toggle with this the tab off.
mac-toggle-tab-bar is a function, not a variable. I have edited your answer accordingly. (The edit is now in the review queue.)