Remove list table "no callback" message

[BrianHenryIE]

This is odd, it was removed in #224 but was still there today.

The bug prevents third parties from adding a custom column to the tickets table, e.g. the Priority column here:

https://github.com/BrianHenryIE/bh-wp-kbs-ticket-priorities

now has —No callback found printed in every row of the table in the Priority column.