Removes unused custom email columns in service_desk_settings

What does this MR do and why?

Removes unused custom email columns in service_desk_settings

The columns have been moved to service_desk_custom_email_credentials

The columns have been introduced in !108017 (merged) and the columns have been moved to service_desk_custom_email_credentials in !114917 (merged). This MR also introduced the ignore_colums method in app/models/service_desk_setting.rb:9.

  ignore_columns %i[
    custom_email_smtp_address
    custom_email_smtp_port
    custom_email_smtp_username
    encrypted_custom_email_smtp_password
    encrypted_custom_email_smtp_password_iv
  ], remove_with: '16.1', remove_after: '2023-05-22'

Changelog: changed

Contributes to &11175

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