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.