If you have created fields that you no longer use, it can be beneficial to deactivate them.
To deactivate leads please do the following:
1. Go to "Settings" and afterwards "Account" in the menu on the left.
2. Go to field groups.
3. Click on the three dots at the right and "Edit" on the fields group the field you want to delete is in (Often you only have one field group called "Default").
4. At the right you can edit it to be inactive: