Don't set MaxContentWidth on the menu bar of a BMenuField

Fixes #9816

It is no longer necessary, or even desirable for us to set the max
content width of the menu bar of a BMenuField now that BMenuItem
truncation and BMenuField sizing are working.

The user may, however, wish to set the max content width of the menu
bar of a BMenuField themselves like so:

menuField->MenuBar()->SetMaxContentWidth(width);

and the Interface Kit will automatically deduct the left and right
margins from the width including the space used by the drop down arrow.

headers/private/interface/BMCPrivate.h

@@ -64,7 +64,7 @@ public:
 private:
 								_BMCMenuBar_&operator=(const _BMCMenuBar_&);
 
-			void				_Init(bool setMaxContentWidth);
+			void				_Init();
 
 			BMenuField*			fMenuField;
 			bool				fFixedSize;