Remove ignores for imported column

What does this MR do and why?

Remove ignores for imported column

This is the third and final step in removing the :imported column from several tables of the DB. This MR removes the ignore_column directives in the model classes as the columns no longer exist in the relevant tables:

design_management_designs
events                    
issues                   
merge_requests           
notes                    
resource_label_events     
resource_milestone_events
resource_state_events   
snippets       

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #460213 (closed)