MAI-UX: Do not allow mapping multiple form elements to same MAI database field

Description

This bug fix removes mapped MAI fields from the mapping field drop-down list, so a Form's fields and MAI database fields can only be mapped 1-to-1.

Steps to reproduce

1. A form1 (contains element1, element2) is dragged and dropped to page1.
2. Map form1 to any Marketo database (e.g., Marketo Database).
3. Open element1 > All Properties mode > Extra field mappings > map to Marketo Database "Email" field.
4. Publish element1 element.
5. Open element2 > All Properties mode > Extra field mappings > map to Marketo Database "Email" field also.
6. Publish element2 element.