To disable the Global Menu, use the following Terminal command. Once done, log out or restart your system for changes to take effect.
sudo apt-get autoremove appmenu-gtk appmenu-gtk3 appmenu-qt
This process is completely revertible and the changes can be undone by re-installing the package from the below given command. Once the package is installed, restart your computer.
sudo apt-get install appmenu-gtk3 appmenu-gtk appmenu-qt