The drop-down list is generated automatically from the entries in your database; you cannot manually remove items from the list. If you want an item to disappear from this list, you need to delete all fields with this name from all entries in your database.
For deleting fields, I'd suggest to use XML Replace or the KPFieldsAdminConsole plugin.
https://keepass.info/help/v2/xml_replace.html
https://keepass.info/plugins.html#kpfieldsadmin
Of course, make a backup first.
Best regards,
Dominik