Want to delete UBUNTU 18.04 LTS from MacBook Air 2017 model
Just perform an erase install. Boot to macOS recovery and then repartition the whole drive and the installer downloads from the internet. 


How to completely reinstall Mac OS X?


My answer here works best if you use internet recovery so you don’t need to mess with making a bootable usb and then get to recovery. 


https://apple.stackexchange.com/a/315885/5472


In addition, there is no benefit to using a bootable USB or local recovery partition over just using internet recovery unless you already have the installer downloaded or a bootable Mac spare and handy. See About macOS Recovery for details.