$ sudo apt-get remove flatpak 

Uninstall flatpak including dependent package

If you would like to remove flatpak and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove flatpak 

Use Purging flatpak

If you use with purge options to flatpak package all the configuration and dependent packages will be removed.

$ sudo apt-get purge flatpak 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove flatpak 

See Also,