Remove unused menu field from macos NavigationToolbar2.

The attribute is never referenced in the entire file; it appears to be a
leftover of old style navigation toolbars, which were removed in 28effb4
and earlier.

Author: anntzer

src/_macosx.m

@@ -865,7 +865,6 @@ - (void)save_figure:(id)sender;
 
 typedef struct {
     PyObject_HEAD
-    NSPopUpButton* menu;
     NSTextView* messagebox;
     NavigationToolbar2Handler* handler;
     int height;