Well, if you don't mind adding a PPA
then yes!
Grub Customizer:
Grub Customizer is a graphical interface to configure the grub2/burg settings.It allows you to edit the GRUB2 menu entries: reorder, rename or add/remove entries.
Installation:
sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer
How to disable Grub2 submenu?
Once installed, Hit Alt+F2, type grub-customizer
and hit Enter.
Select the submenu entries one by one and click on ↑(Up) button in toolbar to move them to main menu.
And no more submenu! Save your settings by clicking on save button in toolbar. That's it!
To remove Grub Customizer run following commands in terminal
sudo apt-get autoremove --purge grub-customizer
sudo add-apt-repository -r ppa:danielrichter2007/grub-customizer
sudo apt-get update