Right now we automatically clear empty fields from the UI upon saving.
That's not a great UX because:

• We force the user to manually delete every written data in all the fields they want to remove.
• It's not intuitive
• We will have some dropdowns with default and disabled fields (date daya nd month dropdown), so users can't select an empty value to clear it.

Adding a simple delete button with a simple X or trashcan icon alongside each field will make it easier to clear and control the data.
Obviously, the data will not be removed unless the user saves the contact, so accidental clicks are not really a data-loss issue.

Try run: https://treeherder.mozilla.org/jobs?repo=try-comm-central&revision=a921b905b80d1e9a716291c3cdf04e30fecb3e63

Linting fixed.

Pushed by alessandro@thunderbird.net:
https://hg.mozilla.org/comm-central/rev/a6ae8a90706c
Implement a remove button for the individual fields in the Address Book edit contact view. r=darktrojan