Revert "src/nsterm.m: fix window tabbing on macOS"

This reverts commit 2207f9adcc.

etc/NEWS

@@ -3234,14 +3234,6 @@ The new variable 'xwidget-webkit-download-dir' says where to download to.
 'module-file-suffix' now has the value ".dylib" on macOS, but the
 ".so" suffix is supported as well.
 
-+++
-** On macOS, Emacs now supports native tabs (in macOS 10.12 and later).
-Native tabbing behavior is specified system-wide via System
-Preferences, under under "General => Prefer tabs when opening
-documents".  In macOS versions 10.15 and earlier, this option is
-located under Dock instead.  Note that this feature is unrelated to
-the Emacs 'tab-bar-mode'.
-
 +++
 ** On MS-Windows, Emacs can now toggle the IME.
 A new function 'w32-set-ime-open-status' can now be used to disable

src/nsterm.m

@@ -7628,6 +7628,14 @@ - (instancetype) initFrameFromEmacs: (struct frame *)f
          selector:@selector (viewDidResize:)
              name:NSViewFrameDidChangeNotification object:nil];
 
+  /* macOS Sierra automatically enables tabbed windows.  We can't
+     allow this to be enabled until it's available on a Free system.
+     Currently it only happens by accident and is buggy anyway.  */
+#ifdef NS_IMPL_COCOA
+  if ([win respondsToSelector: @selector(setTabbingMode:)])
+    [win setTabbingMode: NSWindowTabbingModeDisallowed];
+#endif
+
   ns_window_num++;
   return self;
 }