Deleting Custom ringtones from iPhone / iTunes 12.7
You can delete custom ringtones right on your iPhone using GarageBand. 


Launch GarageBand on your iPhone 
If you’ve never created a song on GarageBand, then you need to create one. It doesn’t matter what you create, you just need to have a song on the “Recents” screen. 
Navigate to the “Recents” screen and long-tap on one of the songs. 
Some floating options will appear 
Select “Share” and then select “Ringtone”. 
If a message appears about ringtones being 30 sec long etc’, just press “Continue”.
You should now be on the export screen. Press on “My Ringtones”.
A list of your custom ringtones will appear. 
Press on “Edit”.
Delete the ringtones you don’t need. 
That’s it. You can now cancel back to the main screen