If an update to an object such as a ticket results in a corresponding update
to the CDATA table, and one or more of the fields have been deleted from the
ticket details form, then, avoid updating the CDATA table for those fields.

This fixes an issue where the CDATA table is dropped and recreated when
fields are added and removed from the form. The table will only contain
fields which are currently on the form when the table is created. Therefore,
deleted fields will never be on the CDATA table and will result in an SQL
error.