Cocoa: Do not process ampersands in menus twice.

When syncing between QAction and native NSMenuItems, the ampersands
(mnemonics) were removed twice. This lead to double ampersands being
removed instead of replace with single ones.

Task-number: QTBUG-37933
Change-Id: If1d9cd247b467472647b22b38460b44b03f13d82
Reviewed-by: Liang Qi <liang.qi@digia.com>