Apple - How to delete/remove custom ringtones (tones) from iPhone in iTunes 12.7?

If you tick the box "Manually manage music and videos", and click the "Apply" button, you'll be able to select a ringtone from your device (in iTunes) and hit your delete key (you'll be prompted for a confirmation to delete the file).

In iTunes on the computer.

1. Select the Device
2. Select Summary
3. Check the box next to Manually manage music and videos.
4. Select Apply

Now you should be able to delete ringtones using iTunes.

Actually, there is no Tone tab in iTunes 12.7, but it still on your iPhone, so you can still use it to delete ringtones from your iPhone directly. Here are the steps:

1. Open iTunes and connect your iPhone.
2. Click on the Device.
3. Scroll down and click on Tones.
4. Select a ringtone, right click and choose Delete from Library.