MT: Stop stamping last_emailed_at on bulk-imported users (#43150)

Previously, the bulk importer set `last_emailed_at` to the import time
on every migrated user to keep digest emails from going out, and that
timestamp then showed customers a "last emailed" date that never
happened.

This change drops the stamp, since the digest job now gates on
`last_seen_at` and no sending path reads `last_emailed_at`, so migrated
users show no email history until they are actually emailed.

script/bulk_import/base.rb

@@ -1463,7 +1463,6 @@ class BulkImport::Base
     user[:staged] = false if user[:staged].nil?
     user[:admin] ||= false
     user[:moderator] ||= false
-    user[:last_emailed_at] ||= NOW
     user[:created_at] ||= NOW
     user[:updated_at] ||= user[:created_at]