Problem:
The user is not able to add a new custom field with a specific slug, because the field already exists and would like to remove it.

Solution:
Note that you can reuse existing custom fields in different field groups, check this screenshot http://prntscr.com/vj57sy

If you need to remove the existing field, go to Toolset->Custom Fields, scroll down and click on "Post Field Control", check this screenshot http://prntscr.com/vj58i3