$ sudo apt-get remove flex-old 

Uninstall flex-old including dependent package

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

$ sudo apt-get remove --auto-remove flex-old 

Use Purging flex-old

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

$ sudo apt-get purge flex-old 

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 flex-old 

See Also,